WO2024094527A1

WO2024094527A1 - Non-human vertebrates & cells

Info

Publication number: WO2024094527A1
Application number: PCT/EP2023/079835
Authority: WO
Inventors: Hanif ALI; Jasper Clube
Original assignee: Quadrucept Bio Limited
Priority date: 2022-11-05
Filing date: 2023-10-25
Publication date: 2024-05-10
Also published as: GB202216503D0

Abstract

The invention relates to novel means to produce protein multimers that are multimerised in vivo using self-associating multimerisation domains (SAMs), such as self-associating tri- or tetramerisation domains.

Description

NON-HUMAN VERTEBRATES & CELLS TECHNICAL FIELD The invention relates to novel means to produce protein multimers that are multimerised in vivo using self-associating multimerisation domains (SAMs), such as self-associating tri- or tetramerisation domains. BACKGROUND Multimers of effector domains have recognized utility in medical and non-medical applications for combining and multiplying the activity and presence of effector domains, eg, to provide for higher avidity of antigen binding (for effector domains that are antibody or TCR binding domains, for example) or for enhancing biological or binding activity, such as for providing bi- or multi-specific targeting or interaction with target ligands in vivo or in vitro. Multimerisation domains which cause self-assembly of protein monomers into multimers are known in the art. Examples include domains found in transcription factors such as p53, p63 and p73, as well as domains found in ion channels such as TRP cation channels. Another example is the tetramerization domain of Transthyretin (TTR TD). The transcription factor p53 can be divided into different functional domains: an N-terminal transactivation domain, a proline-rich domain, a DNA-binding domain, a tetramerization domain and a C-terminal regulatory region. The tetramerization domain of human p53 extends from residues 325 to 356, and has a 4-helical bundle fold (Jeffrey et al., Science (New York, N.Y.) 1995, 267(5203):1498-1502). The TRPM tetramerization domain is a short anti-parallel coiled-coil tetramerization domain of the transient receptor potential cation channel subfamily M member proteins 1-8. It is held together by extensive core packing and interstrand polar interactions (Fujiwara et al., Journal of Molecular Biology 2008, 383(4):854-870). Transient receptor potential (TRP) channels comprise a large family of tetrameric cation-selective ion channels that respond to diverse forms of sensory input. Another example is the potassium channel BTB domain. This domain can be found at the N terminus of voltage-gated potassium channel proteins, where represents a cytoplasmic tetramerization domain (T1) involved in assembly of alpha-subunits into functional tetrameric channels (Bixby et al., Nature Structural Biology 1999, 6(1):38-43). This domain can also be found in proteins that are not potassium channels, like KCTD1 (potassium channel tetramerization domain-containing protein 1; Ding et al., DNA and Cell Biology 2008, 27(5):257-265). Multimeric antibody fragments have been produced in vitro using a variety of multimerisation techniques, including biotin, dHLX, ZIP and BAD domains, as well as p53 (Thie et al., Nature Biotech., 2009:26, 314-321). Biotin, which is efficient in production, is a bacterial protein which induces immune reactions in humans. US11,453,726 discloses multimers, such as tetramers, of polypeptides that are multimerised using self-associating multimerisation domains (SAMs). In vitro expression of polypeptides comprising SAMs and assembly into useful multimers is disclosed. SUMMARY OF THE INVENTION The invention provides:- In a First Configuration A non-human vertebrate (eg, a mouse or a rat), wherein the genome of the vertebrate comprises a gene locus that encodes a polypeptide, wherein the locus comprises a first nucleotide sequence for producing a first effector moiety (EM1) and a second nucleotide sequence encoding a self-associating multimerisation domain (SAM), wherein the locus is capable of producing copies of a polypeptide comprising EM1 and SAM, wherein the SAM domains of the polypeptides associate together to produce a polypeptide multimer comprising a plurality of EM1 moieties. In a First Aspect:- A non-human vertebrate (eg, a mouse or a rat), wherein an immunoglobulin (Ig) locus of the genome of the vertebrate comprises a first nucleotide sequence for producing a first effector moiety (EM1) and a second nucleotide sequence encoding a self-associating multimerisation domain (SAM), wherein the locus is capable of producing copies of a polypeptide comprising EM1 and SAM, wherein the SAM domains of the polypeptides associate together to produce a polypeptide multimer comprising a plurality of EM1 moieties. Preferably, the SAM does not comprise an antibody CH2 or CH3 domain. In a preferred embodiment, the SAM is a self-associating tetramerization domain (TD). Preferably, the multimer comprises 4 copies of said polypeptide. Advantageously (as exemplified by Examples 2 onwards) the locus may comprise in 5’ to 3’ order an Ig intronic enhancer (eg, Eμ enhancer or Eiκ) and a second region (eg, a constant region), wherein the second region comprises the second nucleotide sequence encoding the SAM. Preferably, in an embodiment the locus of the invention is an Ig locus comprising in 5’ to 3’ order a first region (eg, a variable region), an intron comprising an intronic enhancer (eg, Eμ enhancer or Eiκ) and a second region (eg, a constant region), wherein the second region comprises a second nucleotide sequence encoding a self-associating multimerisation domain (SAM). By placing the SAM-encoding sequence downstream of the enhancer, mutation of SAM is usefully minimised. The invention in some embodiments advantageously enables the placement of elements that take advantage of natural mutation capacity of lymphocytic cells (B-cells or T-cells), and other elements (such as a SAM and one or more predetermined effector moieties) can be carefully positioned to minimise mutation, whereby a repertoire of polypeptides and multimers differing in their effector moieties can be obtained and usefully screened using conventional screening techniques. Furthermore, by bringing these features together in a non- human vertebrate, such as a mouse, it is possible to harness in vivo selection and expressibility of multimers by the vertebrate’s natural immunoglobulin development systems. Thus, those multimers of the repertoire are multimers that have successfully undergone mutation, expression and in vivo selection (and thus successfully feature in lymphocytic cells and binding ligand repertoire of the vertebrate). This is useful to increase the chances of the multimers having good product developability and biophysical characteristics, such as for medicament or other product manufacture. Thus, the invention in Aspects provides the following. In Third Aspect:- A non-human vertebrate comprising a plurality of lymphocytic cells, the cells each comprising an Ig locus, wherein the locus comprises in 5’ to 3’ order a first region, an intron comprising an intronic enhancer (eg, Eμ enhancer or Eiκ) and a second region, wherein the first region comprises a first nucleotide sequence that encodes a first effector moiety (EM1) and the second region comprises a second nucleotide sequence encoding a self-associating multimerisation domain (SAM), wherein the locus is capable of producing copies of a polypeptide comprising EM1 and SAM, wherein the SAM domains of the polypeptides associate together to produce a polypeptide multimer comprising a plurality of EM1 moieties, wherein the vertebrate is capable of producing a repertoire of said multimers that comprise different EM1 moieties. In a Fourth Aspect:- A non-human vertebrate comprising a plurality of B-cells, the cells each comprising an antibody locus, wherein the locus comprises in 5’ to 3’ order a first region, an intron comprising an intronic enhancer (eg, Eμ enhancer or Eiκ) and a second region, wherein the first region comprises a first nucleotide sequence that encodes a first effector moiety (EM1) and the second region comprises a second nucleotide sequence encoding a self-associating multimerisation domain (SAM), wherein the locus is capable of producing copies of a polypeptide comprising EM1 and SAM, wherein the SAM domains of the polypeptides associate together to produce a polypeptide multimer comprising a plurality of EM1 moieties, wherein the vertebrate is capable of producing a repertoire of said multimers that comprise different EM1 moieties. A non-human vertebrate comprising a plurality of T-cells, the cells each comprising a TCR locus, wherein the locus comprises in 5’ to 3’ order a first region, an intron comprising an intronic enhancer (eg, TCRδ enhancer) and a second region, wherein the first region comprises a first nucleotide sequence that encodes a first effector moiety (EM1) and the second region comprises a second nucleotide sequence encoding a self-associating multimerisation domain (SAM), wherein the locus is capable of producing copies of a polypeptide comprising EM1 and SAM, wherein the SAM domains of the polypeptides associate together to produce a polypeptide multimer comprising a plurality of EM1 moieties, wherein the vertebrate is capable of producing a repertoire of said multimers that comprise different EM1 moieties. In a Second Configuration A non-human vertebrate or a non-human vertebrate cell that comprises a rearranged Ig variable region (eg, an VH or VL region) encoding a V domain, wherein the variable region is ectopically positioned in the genome of the vertebrate or cell, wherein the vertebrate or cell expresses polypeptides each comprising a SAM and a said V domain, wherein the V domains of the polypeptides comprise most commonly a CDR3 length of 11 to 16 (eg, 15) amino acids. In a Third Configuration In a First Aspect:- A cell or a plurality of cells, wherein each cell is a) an isolated lymphocytic cell (optionally a B- or T-cell) obtainable from a vertebrate of any Configuration, each comprises a locus as defined in any preceding claim for producing polypeptides comprising EM1 and a SAM.; or b) an embryonic stem cell or pluripotent stem cell that can develop into a vertebrate according to any Configuration. In a Second Aspect:- A B-cell comprising an antibody locus, wherein the locus comprises in 5’ to 3’ order a first region (eg, a variable region), an intron comprising an intronic enhancer (eg, Eμ enhancer or Eiκ) and a second region (eg, a constant region), wherein the first region comprises a first nucleotide sequence that encodes a first effector moiety (EM1) and the second region comprises a second nucleotide sequence encoding a self-associating multimerisation domain (SAM), wherein the locus is capable of producing copies of a polypeptide comprising EM1 and SAM, wherein the SAM domains of the polypeptides associate together to produce a polypeptide multimer comprising a plurality of EM1 moieties. In a Fourth Configuration A lymphocyte cell obtainable from the vertebrate of any Configuration and comprising a locus for expressing a polypeptide, wherein the locus comprises (in 5' to 3' direction) (a) an unrearranged Ig variable region comprising at least one V gene segment, optionally at least one D gene segment, and at least one J gene segment, wherein the variable region is capable of rearrangement for expressing a rearranged Ig variable domain; (b) an Ig locus intronic enhancer; and (c) a first Ig constant region encoding a first C domain; wherein the locus comprises a nucleotide sequence encoding a first self-associating mulimerization domain (SAM) (such as a self-associating tetramerization domain (TD)), the locus being operable to express a polypeptide comprising said rearranged V domain, SAM and C domain. In a Fifth Configuration A method of producing a nucleic acid for expressing a polypeptide comprising an EM1 and SAM, the method comprising (a) determining a nucleotide sequence of an EM1-encoding sequence comprised by a locus of a vertebrate of any Configuration, wherein EM1 is a V domain and the first nucleotide sequence comprises a rearranged V region encoding said V domain; and (b) producing a nucleic acid by operably connecting the sequence of step (a) with a nucleotide sequence encoding a SAM. In a Fifth Configuration A multimer comprising a first, a second, a third and a fourth copy of the polypeptide comprising an EM1 and a TD, wherein EM1 is an Ig variable domain comprising an amino acid sequence that is encoded by the first nucleotide sequence of a vertebrate according to any Configuration wherein the first nucleotide sequence comprises an Ig V region that has been rearranged in the vertebrate, wherein the vertebrate is a mouse and the amino acid sequence is encoded by a nucleotide sequence that comprises mouse pattern activation-induced cytidine deaminase (AID) and/or terminal deoxynucleotidyl transferase (TdT) mutation, optionally wherein the polypeptide comprises one or more human constant domains (such as an Fc region). In a Sixth Configuration A polyclonal polypeptide multimer population, wherein each multimer is a multimer of at least 3 or 4 copies of a respective polypeptide comprising an Ig V domain and a SAM, wherein SAMs comprised by each multimer are associated together, and wherein the population comprises at least 10 different types of said V domain of the polypeptides. In a Seventh Configuration A method for identifying or obtaining an antibody variable domain, a nucleotide sequence encoding an antibody variable domain, or an expression vector or host cell that is capable of expressing the domain, wherein the domain is capable of specifically binding to a target antigen, the method comprising (a) contacting the population of the Sixth Configuration with said antigen; (b) binding multimers comprised by said population to said antigen; and (c) isolating or identifying one or more multimers that bind to the antigen, or isolating or identifying a said V domain of such a multimer; or identifying a nucleotide sequence encoding a V domain of such a multimer; and (d) optionally connecting the sequence to a nucleotide sequence encoding a SAM to produce a nucleic acid for expressing a polypeptide comprising the V domain and SAM; and optionally expressing and isolating said polypeptide or multimers thereof. In a Eighth Configuration A non-human vertebrate (eg, mouse) blastocyst or pre-morula embryo implanted with an ES or iPS cell comprising a locus of the invention, wherein the blastocyst or pre-morula embryo implanted with an ES or iPS cell is capable of developing into a vertebrate of any Configuration. In a Ninth Configuration A method of obtaining a humanised and affinity matured antigen-specific antibody heavy chain or a multimer thereof, the method comprising producing the heavy chain in vivo in a non-human vertebrate (eg, a mouse or a rat) comprising a functional activation induced cytidine deaminase (AID) by immunising the vertebrate with the antigen and obtaining somatic hypermutation and isotype switching in a B-cell of the vertebrate from an endogenous mu isotype to a human non-mu isotype, wherein an affinity matured antigen- specific antibody heavy chain is produced and expressed by the vertebrate, the non-mu constant domains of the heavy chain being human constant domains encoded by a non-mu constant region of the vertebrate and the heavy chain comprising a SAM encoded by the non- mu constant region, wherein copies of the heavy chain self-associated via SAMs to produce multimers and the method further comprises isolating at least one said multimer. In a Tenth Configuration A nucleic acid (eg, a YAC) comprising a transgene for microinjection into a non-human vertebrate ES cell, the transgene comprising a locus of the invention. In a Eleventh Configuration A nucleic acid vector comprising a nucleic acid, wherein the nucleic acid comprises homology arms for recombination with the genome of a non-human vertebrate ES cell, wherein a nucleotide sequence encoding a SAM is between the homology arms, whereby said recombination is capable of inserting the nucleotide sequence into an endogenous Ig locus of the cell 3’ of an intronic enhancer thereof. The invention also provides host cells, a method of making an antibody multimer, a method of producing a polypeptide comprising EM1 and a SAM, a method of producing a multimer of copies of a polypeptide comprising EM1 and a SAM, a method of obtaining a nucleotide sequence encoding an EM1, a method for producing an expression vector for expressing a polypeptide comprising EM1 and a SAM BRIEF DESCRIPTION OF THE DRAWINGS Figure 1. Schematic representation of V(D)J recombination and IgM production and class switching to generate IgG1 antibodies. Figure 2. Schematic representation of V(D)J recombination and IgM production and class switching to generate multivalent IgG1 antibody multimers. Figure 3. Schematic representation of the targeting strategy to modify ES cells with introduced TD domain linked to the C-terminus of Cγ1 constant region. Figure 4. Schematic representation of IgH loci with introduced TD domain linked to C- terminus of all IgH constant domains capable of producing multivalent IgD, IgG, IgE and IgA antibodies. DETAILED DESCRIPTION We have realised the advantage of producing a single type of polypeptide comprising an EM1 and SAM per cell. For example, the invention relates to a vertebrate comprising lymphocytes (B-cells or T-cells) wherein each cell produces only a single, respective type of said polypeptide, ensuring that only one type of multimer per said lymphocytic cell. This is useful, since consequently all multimers of the invention produced by each said cell will only comprise polypeptides of said single type (where the polypeptides are associated by SAMs). This opens up the possibility of using a non-human vertebrate to produce repertoires of different multimers (such as tetramers of a polypeptide) wherein a desired, selected multimer can be traced back to a single cell genome encoding a single polypeptide species of the invention. With this ability, it is possible to screen repertoires of multimers of the invention quickly in large numbers and using conventional antibody or TCR screening techniques (such as using conventional in silico screening methods for determining nucleotide sequences of desired binding domains, EM1s). By compartmentalising the polypeptides of the invention in cells, such as B-cells or T-cells, this opens up also the advantage of being able to generate a repertoire of multimers that are multi-specific for antigen binding (eg, bi-specific multimers) straight forwardly in the cells whilst avoiding prior art issues of chain pairing inefficiencies and consequent yield loss (undesired pairing of different antibody chains, for example, giving low yields of multi- specific products). Instead, the locus is dedicated to producing a single type of polypeptide of the invention and this may itself comprise a plurality of antigen binding sites that differ by the antigen to which they specifically bind. Thus, all multimers produced by a particular cell of the invention are always multimers of a single type of polypeptide (copies of the polypeptide being associated together by SAMs). This reliably enables traceability of a multimer back to a cell that encodes its cognate polypeptide. Using standard cloning and expression in vitro a polypeptide comprising the EM1 from the desired multimer that was selected from the vertebrate B- or T-cell repertoire can be combined with the SAM used by the vertebrate or a different SAM. For example, after selecting a polypeptide from the vertebrate, the EM1 of the polypeptide can be sequenced (and used to produce a cognate nucleotide sequence encoding the EM1) or the selected EM1 sequence can be traced to a nucleotide sequence of a B-cell or T-cell of the vertebrate and an expression vector comprising the nucleotide sequence and a sequence encoding the original SAM (ie, the one that was comprised with EM1 in the selected polypeptide) or a different SAM can be constructed using standard recombinant DNA technology, whereby the resulting vector can express a polypeptide comprising EM1 with a SAM and copies of the polypeptide self-associate to produce a multimer comprising a plurality of copies of EM1. The resultant polypeptide has many potential industrial applications, such as a medicament or other applications where antigen binding products are conventionally used. It is particularly advantageous in this polypeptide for the SAMs to be SAMs of the same type that were comprised by the polypeptide selected from the vertebrate. This because the vertebrate would have already pre-selected a repertoire of polypeptides comprising EM1 and SAMs that can successfully form multimers (such as tetramers) in in vivo systems and can be successfully expressed by eukaryotic cells (such as B-cells) in functional form that can bind to cognate antigen. Thus, with this knowledge, it is useful to preserve the combination of this EM1 and SAM when producing the expression vector, whereby the encoded polypeptides also can be expressed (eg, in vitro) from the vector in functional multimer form that binds to cognate antigen. By comprising multiple copies of EM1, the polypeptides herein can bind to cognate antigen with higher avidity than is possible by a single EM1. Thus, for example, EM1 may have a relatively low affinity binding strength for its cognate antigen, but a multimer of the invention – by comprising multiple copies of EM1 – has a higher binding strength for cognate antigen. The presence of multiple EM1s therefore provides for higher avidity for antigen binding. Any repertoire herein (eg, a repertoire of members that are EM1s, polypeptides, multimers or cells) may comprise at least 10², 10³, 10⁴, 10⁵, 10⁶, 10⁷, 10⁸, 10⁹, 10¹⁰, 10¹¹, 10¹², 10¹³ or 10¹⁴ of said members. Any repertoire herein (eg, a repertoire of members that are EM1s, polypeptides, multimers or cells) may comprise at least 10², 10³, 10⁴, 10⁵, 10⁶, 10⁷, 10⁸, 10⁹, 10¹⁰, 10¹¹, 10¹², 10¹³ or 10¹⁴ of different types of said members. Thus, in a First Configuration, the invention provides:- A non-human vertebrate (eg, a mouse or a rat), wherein the genome of the vertebrate comprises a gene locus that encodes a polypeptide, wherein the locus comprises a first nucleotide sequence for producing a first effector moiety (EM1) and a second nucleotide sequence encoding a self-associating multimerisation domain (SAM), wherein the locus is capable of producing copies of a polypeptide comprising EM1 and SAM, wherein the SAM domains of the polypeptides associate together to produce a polypeptide multimer comprising a plurality of EM1 moieties. There is also provided:- A non-human vertebrate (eg, a mouse or a rat), wherein an immunoglobulin (Ig) locus of the genome of the vertebrate comprises a first nucleotide sequence for producing a first effector moiety (EM1) and a second nucleotide sequence encoding a self-associating multimerisation domain (SAM), wherein the locus is capable of producing copies of a polypeptide comprising EM1 and SAM, wherein the SAM domains of the polypeptides associate together to produce a polypeptide multimer comprising a plurality of EM1 moieties. Advantageously, (as supported by Examples 2 onwards), in embodiments the locus comprises in 5’ to 3’ direction an Ig intronic enhancer (eg, Eμ or Eiκ) and said second nucleotide sequence (and optionally the third sequence encoding EM2 as described below). As shown in the Examples, this placement of these sequence(s) may advantageously minimise somatic hypermutation during B-cell development in the vertebrate. This can usefully be borne in mind when deciding where to position the sequence encoding SAM in an Ig locus, with positioning of that sequence 3’ of the intronic enhancer in the locus appearing to be very beneficial to maintain the SAM sequence and thus function to self-multimerise to form multimers in the mouse of the invention. Similarly, placement of a sequence encoding EM2 or any other predetermined moiety 3’ of the intronic enhancer may be beneficial to minimise mutation of this desired moiety in the resultant multimers. An enhancer herein may be an intronic enhancer, eg, an antibody heavy chain, kappa chain or lambda chain intronic enhancer. An enhancer may be a human or mouse Eμ, human or mouse Eiκ, mouse Eλ3-1 or mouse Eλ2-4 enhancer. For example, the locus is an antibody heavy chain locus and the enhancer is a Eμ. For example, the locus is a vertebrate of a non-human species (eg, mouse) antibody heavy chain locus and the enhancer is a human Eμ or Eμ of said species. For example, the locus is a mouse antibody heavy chain locus and the enhancer is a mouse Eμ. For example, the vertebrate is a mouse locus is an endogenous mouse antibody heavy chain locus and the enhancer is a mouse (eg, endogenous) Eμ. For example, the locus is a vertebrate of a non-human species (eg, mouse) antibody kappa chain locus and the enhancer is a human Eiκ or Eiκ of said species. For example, the locus is a mouse antibody kappa chain locus and the enhancer is a mouse Eiκ. For example, the vertebrate is a mouse locus is an endogenous mouse antibody kappa chain locus and the enhancer is a mouse (eg, endogenous) Eiκ. The locus may be an antibody heavy chain locus, an antibody light chain locus (eg, a kappa or lambda locus) or a TCR locus (eg, an alpha, beta, gamma or delta locus). In an alternative, the locus may be a transgene, such as a transgene that has been randomly inserted into the genome of the vertebrate. Preferably, the multimer comprises at least 3 or 4 copies of said polypeptide. For example, the multimer is a polypeptide tetramer. In an example, the locus comprises in 5' to 3' orientation the first nucleotide sequence and the second nucleotide sequence (and optionally a further nucleotide sequence encoding a further effector moiety). In an example, the locus comprises in 5' to 3' orientation the second nucleotide sequence and the first nucleotide sequence (and optionally a further nucleotide sequence encoding a further effector moiety, wherein the further sequence is upstream of (5’ of) the second sequence). In an embodiment, the effector moieties are capable of binding the same antigen. In an embodiment, the effector moieties are capable of binding different antigens. Thus, the latter is useful for producing multi-specific (eg, bi-specific) multimers. Optionally, each effector moiety is a peptide or protein domain, eg, an immunoglobulin variable domain, such as an antibody or TCR variable domain. EM1 may be, for example, an antibody single variable domain that is capable of binding a cognate antigen. EM1 may be, for example, an antibody single VH domain that is capable of binding a cognate antigen. EM1 may be, for example, an antibody single VL (eg, Vκ or Vλ) domain that is capable of binding a cognate antigen. Optionally, the first nucleotide sequence is an Ig locus variable region. The vertebrate or cell is capable of producing a multimer of copies of said polypeptide wherein the copies are associated together by SAMs. The polypeptides associate together by self-association of the SAMs comprised by the polypeptides to produce the polypeptide multimer. Optionally, the SAM is a trimerisation domain, eg, PLAD domain (the pre-ligand binding assembly domain), IZ (CN4-based isoleucine zipper) or foldon (the natural trimerization domain of T4 fibritin). Most preferably, the SAM is a tetramerization domain (also referred to as a TD herein). Preferably, the SAM is not comprised by an antibody Fc region. Preferably, the SAM does not comprise an antibody CH2 or CH3 domain. Preferably, the SAM does not comprise an Ig constant domain. Preferably, the SAM is a tetramerization domain (TD) wherein the multimer comprises 4 (and no more than 4) copies of said polypeptides. The vertebrate may be a mammal. The vertebrate may be a rodent, eg, a mouse or a rat. The vertebrate may be a mouse, rat, rabbit, guinea pig, chicken, fish, bird (eg, chicken), reptile, Camelid (eg, llama), cow, chimpanzee or non-human primate. Preferably, the vertebrate is a mouse. In an embodiment, the vertebrate is a rat. The genome of one or more B-cells of the vertebrate may comprise said locus, such as when EM1 is an antibody heavy chain variable domain, eg, a nanobody. The genome of one or more T-cells of the vertebrate may comprise said locus. Optionally, the germline of the vertebrate comprises said locus. In another option, the vertebrate is a chimaera comprising first cells that do not comprise the locus and second cells that do comprise the locus. For example, the vertebrate is a chimaera comprising first B-cells that do not comprise the locus and second B-cells that do comprise the locus. For example, when the locus of the invention is an antibody locus, B-cells of the vertebrate comprise the locus and endogenous antibody chain production is inactivated (eg, endogenous heavy chain production is inactivated, eg, endogenous heavy and light chain production is inactivated). The skilled addressee knows conventional techniques for achieving such inactivation, eg, J region deletion from endogenous heavy chain loci. In an embodiment, the locus of the invention is a randomly-integrated locus comprised by the genome of the cell or vertebrate. In an embodiment, the locus of the invention is a locus produced by targeted insertion of heterologous DNA into an endogenous Ig locus of the genome of the cell or vertebrate using known techniques. In an embodiment, the locus of the invention is a randomly-integrated antibody (eg, heavy chain) locus comprised by the genome of the cell or vertebrate. Microinjection of the transgene into a non-human vertebrate ES cell (eg, a mouse ES cell) can be used to introduce the transgene into the cell genome. In an embodiment, the antibody locus of the invention is a locus produced by targeted insertion of heterologous heavy chain variable region DNA and said SAM-encoding sequence into an endogenous Ig locus of the genome of the cell or vertebrate. For example, the SAM-encoding sequence is inserted into a constant region of the endogenous locus and the V region DNA is operably inserted in the endogenous locus upstream of said constant region, wherein the locus is capable of encoding a polypeptide comprising a V domain (ie, an EM1) and a SAM, whereby copies of the polypeptide are capable of self-associating to produce a multimer comprising a plurality of EM1. In a first example, the V region DNA is unrearranged antibody heavy chain V region DNA. Preferably, the unrearranged DNA comprises at least one human VH gene segment, at least one human D gene segment and at least one JH gene segment, wherein the segments are capable of rearranging to produce a rearranged VH region encoding EM1 in the vertebrate. In a second example, the V region DNA is unrearranged light chain (eg, kappa or lambda) V region DNA. Preferably, the unrearranged DNA comprises at least one human VL gene segment and at least one human JL gene segment (eg, a human Vκ and a human Jκ; or a human Vλ and a human Jλ), wherein the segments are capable of rearranging to produce a rearranged VL region encoding EM1 in the vertebrate. Alternatively, the V region DNA is rearranged VH or VL region DNA encoding a human V domain, wherein the locus is capable of producing a polypeptide comprising EM1 and a SAM. The rearranged V domain may be subject to hypermutation in the vertebrate, whereby a repertoire of polypeptides are produced that differ in their EM1s. By routine screening of the repertoire, one can select a desired multimer, polypeptide or EM1. In an embodiment, the V region DNA is inserted into an antibody locus 3’ of an intronic enhancer, eg, VH region DNA is inserted downstream of Eμ in an endogenous heavy chain locus of the vertebrate or cell. In this way, hypermutation of the V region (such as a rearranged V region) can be avoided so that a predetermined EM1 is predictably encoded. In one embodiment, the first nucleotide sequence comprises a substantially complete human repertoire of human VH, D and JH gene segments. For example, in this embodiment the first nucleotide sequence comprises a complete human repertoire of human VH, D and JH gene segments. The first nucleotide sequence may comprise human VH6-1. In one embodiment, the first nucleotide sequence comprises functional human VH gene segments from VH7-4-1 to VH6-1; or from VH2-5 to VH6-1; or from VH3-23 to VH6-1; or from VH4-39 to VH6-1; or from VH3-43D to VH6-1; or from VH3-64 to VH6-1. The skilled addressee will readily understand that by “functional human VH gene segments” is meant VH gene segments that are capable of recombining with a D and J to produce a rearranged V region encoding a V domain; this excludes, therefore, pseudogene VHs. Additionally or alternatively, the first nucleotide sequence may comprise human D1-26. Additionally or alternatively, the first nucleotide sequence may comprise human D gene segments from D3-22 to D1-26; or from D2-15 to D1-26; or from D2-8 to D1-26; or from D1-1 to D1-26. Additionally or alternatively, the first nucleotide sequence may comprise human JH6. Additionally or alternatively, the first nucleotide sequence may comprise human JH gene segments from JH1- JH6. The first nucleotide sequence may comprise human Vκ1-39 or Vκ3-20. In one embodiment, the first nucleotide sequence comprises functional human VH gene segments from Vκ1-8 to Vκ4-1; or from Vκ2-24 to Vκ4-1; or from Vκ2-40 to Vκ4-1; or from Vκ3D-7 to Vκ4-1. Additionally or alternatively, the first nucleotide sequence may comprise human Jκ gene segment Jκ1, Jκ2, Jκ3, Jκ4 or Jκ5. Additionally or alternatively, the first nucleotide sequence may comprise human Jκ gene segments from Jκ1 to Jκ5. Optionally, the third nucleotide sequence comprises a substantially complete human repertoire of human Vκ and Jκ gene segments. In this option, the third nucleotide sequence comprises a complete human repertoire of human Vκ and Jκ gene segments. In an alternative option, the third nucleotide sequence comprises a substantially complete human repertoire of human Vλ and Jλ gene segments. In this option, the third nucleotide sequence comprises a complete human repertoire of human Vλ and Jλ gene segments. In an alternative option, the third nucleotide sequence comprises a substantially complete human repertoire of human VH, D and JH gene segments. Optionally, the third nucleotide sequence comprises a substantially complete human. In this option, the third nucleotide sequence comprises a complete human repertoire of human VH, D and JH gene segments. In an option, the first nucleotide sequence comprises said human VH, D and JH gene segments and EM2 is a VL domain that is capable of pairing with a VH domain produced by the recombination of the VH, D and J gene segments of the first nucleotide sequence to form a VH/VL binding site that is capable of binding to an antigen. In one embodiment, the third nucleotide sequence comprises a substantially complete human repertoire of human VH, D and JH gene segments. For example, in this embodiment the third nucleotide sequence comprises a complete human repertoire of human VH, D and JH gene segments. Optionally, the first nucleotide sequence comprises a substantially complete human repertoire of human Vκ and Jκ gene segments. In this option, the first nucleotide sequence comprises a complete human repertoire of human Vκ and Jκ gene segments. In an alternative option, the third nucleotide sequence comprises a substantially complete human repertoire of human Vλ and Jλ gene segments. In this option, the first nucleotide sequence comprises a complete human repertoire of human Vλ and Jλ gene segments. In an alternative option, the first nucleotide sequence comprises a substantially complete human repertoire of human VH, D and JH gene segments. Optionally, the first nucleotide sequence comprises a substantially complete human. In this option, the first nucleotide sequence comprises a complete human repertoire of human VH, D and JH gene segments. In an option, the third nucleotide sequence comprises said human VH, D and JH gene segments and EM1 is a VL domain that is capable of pairing with a VH domain produced by the recombination of the VH, D and J gene segments of the third nucleotide sequence to form a VH/VL binding site that is capable of binding to an antigen. Optionally, the expresses antibody heavy chains variable domains (which are EM1 moieties), and wherein at least 70 or 80% (for example, at least 70, 75, 80, 84, 85, 90, 95, 96, 97, 98 or 99%, or 100%) of the variable domains of the heavy chains expressed by the vertebrate are derived from recombination of human heavy chain variable region gene segments comprised by the first nucleotide sequence. Optionally, the expresses antibody light chains variable domains (which are EM1 moieties), and wherein at least 70 or 80% (for example, at least 70, 75, 80, 84, 85, 90, 95, 96, 97, 98 or 99%, or 100%) of the variable domains of the light chains expressed by the vertebrate are derived from recombination of human light chain variable region gene segments comprised by the first nucleotide sequence. Optionally, the expresses antibody kappa light chains variable domains (which are EM1 moieties), and wherein at least 70 or 80% (for example, at least 70, 75, 80, 84, 85, 90, 95, 96, 97, 98 or 99%, or 100%) of the variable domains of the kappa light chains expressed by the vertebrate are derived from recombination of human kappa light chain variable region gene segments comprised by the first nucleotide sequence. Optionally, the expresses antibody lambda light chains variable domains (which are EM1 moieties), and wherein at least 70 or 80% (for example, at least 70, 75, 80, 84, 85, 90, 95, 96, 97, 98 or 99%, or 100%) of the variable domains of the lambda light chains expressed by the vertebrate are derived from recombination of human lambda light chain variable region gene segments comprised by the first nucleotide sequence. Preferably, the vertebrate is capable of expressing a repertoire of at least 100; 1000; 10,000 or 1000,000 different types of the polypeptide of the invention, wherein the types comprise different EM1 moieties. Preferably, the vertebrate is capable of expressing a repertoire of at least 10² to 10⁹ different types of the polypeptide of the invention, wherein the types comprise different EM1 moieties. In an example, all of the different EM1 moieties are produced by recombination of a common set of Ig (eg, antibody) variable region gene segments and hypermutation in the vertebrate. In an example, all of the different EM1 moieties are produced by a common first nucleotide sequence (eg, a sequence encoding a rearranged antibody or TCR variable domain; or a sequence encoding a peptide) and hypermutation of the first sequence in the vertebrate. Advantageously, in embodiments of the invention, the locus of the invention is an Ig locus comprising in 5’ to 3’ order a first region (eg, a variable region), an intron comprising an intronic enhancer (eg, Eμ enhancer or Eiκ) and a second region (eg, a constant region), wherein the second region comprises a second nucleotide sequence encoding a self- associating multimerisation domain (SAM). By placing the SAM-encoding sequence downstream of the enhancer, mutation of SAM is usefully minimised. Advantageously, in embodiments of the invention, the locus of the invention is a antibody locus comprising in 5’ to 3’ order a first region (eg, a variable region), an intron comprising an intronic enhancer (eg, Eμ enhancer or Eiκ) and a second region (eg, a constant region), wherein the first region comprises the first nucleotide sequence that encodes EM1 and the second region comprises a second nucleotide sequence encoding a self-associating multimerisation domain (SAM). Optionally, when a third nucleotide sequence is present (encoding EM2), this may be comprised by the second region. In this case, by placement downstream of the enhancer, this minimises the chances of the third sequence being mutated during B-cell development and maturation in vivo in the vertebrate. Thus, one can obtain polypeptides comprising a repertoire of EM1s (produced by recombination and/or mutation in B-cells of the vertebrate that comprise the locus) wherein the polypeptides comprise a common and predetermined EM2 (and SAM). Also, by placing the SAM-encoding sequence downstream of the enhancer, mutation of SAM is minimised. Thus, the invention provides:- A B-cell comprising an antibody locus, wherein the locus comprises in 5’ to 3’ order a first region (eg, a variable region), an intron comprising an intronic enhancer (eg, Eμ enhancer or Eiκ) and a second region (eg, a constant region), wherein the first region comprises a first nucleotide sequence that encodes a first effector moiety (EM1) and the second region comprises a second nucleotide sequence encoding a self-associating multimerisation domain (SAM), wherein the locus is capable of producing copies of a polypeptide comprising EM1 and SAM, wherein the SAM domains of the polypeptides associate together to produce a polypeptide multimer comprising a plurality of EM1 moieties. A T-cell comprising a TCR locus, wherein the locus comprises in 5’ to 3’ order a first region, an intron comprising an intronic enhancer (eg, TCRα, TCRβ TCRγ or TCRδ enhancer) and a second region, wherein the first region comprises a first nucleotide sequence that encodes a first effector moiety (EM1) and the second region comprises a second nucleotide sequence encoding a self-associating multimerisation domain (SAM), wherein the locus is capable of producing copies of a polypeptide comprising EM1 and SAM, wherein the SAM domains of the polypeptides associate together to produce a polypeptide multimer comprising a plurality of EM1 moieties, wherein the vertebrate is capable of producing a repertoire of said multimers that comprise different EM1 moieties. A non-human vertebrate comprising a plurality of B-cells, the cells each comprising an antibody locus, wherein the locus comprises in 5’ to 3’ order a first region (eg, a variable region), an intron comprising an intronic enhancer (eg, Eμ enhancer or Eiκ) and a second region (eg, a constant region), wherein the first region comprises a first nucleotide sequence that encodes a first effector moiety (EM1) and the second region comprises a second nucleotide sequence encoding a self-associating multimerisation domain (SAM), wherein the locus is capable of producing copies of a polypeptide comprising EM1 and SAM, wherein the SAM domains of the polypeptides associate together to produce a polypeptide multimer comprising a plurality of EM1 moieties, wherein the vertebrate is capable of producing a repertoire of said multimers that comprise different EM1 moieties. A non-human vertebrate comprising a plurality of T-cells, the cells each comprising a TCR locus, wherein the locus comprises in 5’ to 3’ order a first region, an intron comprising an intronic enhancer (eg, TCRα, TCRβ TCRγ or TCRδ enhancer) and a second region, wherein the first region comprises a first nucleotide sequence that encodes a first effector moiety (EM1) and the second region comprises a second nucleotide sequence encoding a self- associating multimerisation domain (SAM), wherein the locus is capable of producing copies of a polypeptide comprising EM1 and SAM, wherein the SAM domains of the polypeptides associate together to produce a polypeptide multimer comprising a plurality of EM1 moieties, wherein the vertebrate is capable of producing a repertoire of said multimers that comprise different EM1 moieties. In an example, the first nucleotide sequence comprises one, more or all human VH gene segments selected from the group consisting of VH3-23, VH7-4-1, VH4-4, VH1-3, VH3-13, VH3-7, VH3-20 and VH3-9. For example, the nucleotide sequence comprises all of these VH gene segments. In an example, the first nucleotide sequence comprises VH3-23, eg, VH3- 23*01 or *04. In an example, the first nucleotide sequence comprises one, more or all human VH gene segments selected from the group consisting of VH3-23*04, VH7-4-1*01, VH4- 4*02, VH1-3*01, VH3-13*01, VH3-7*01, VH3-20*01 and VH3-9*01. For example, the nucleotide sequence comprises all of these VH gene segments. In an example, the first nucleotide sequence comprises one, more or all human V_K gene segments selected from the group consisting of V_K4-1, V_K2-28, V_K1 D-13, V_K1-12, V_K1 D- 12, V_K3-20,V_K1-17,V_K1D-39, V_K3-11, V_K1D-16 and V_K1-9. For example, the nucleotide sequence comprises all of these Vκ gene segments. In an example, the first nucleotide sequence comprises one, more or all human V_K gene segments selected from the group consisting of V_K4-1, V_K2-28, V_K1-13, V_K1-12, V_K1-12, V_K3-20,V_K1-17,V_K1-39, V_K3-11, V_K1-16 and V_K1-9. For example, the nucleotide sequence comprises all of these Vκ gene segments. In an example, the first nucleotide sequence comprises V_K3-20 and/or V_K1-39. In an example, the first nucleotide sequence comprises one, more or all human V_K gene segments selected from the group consisting of V_K4-1*01, V_K2-28*01, V_K1 D-13*d01, V_K1- 12*01, V_K1 D-12*02, V_K3-20*01,V_K1-17*01,V_K1D-39*01, V_K3-11*01, V_K1 D-16*01 and V_K1-9*d01. For example, the nucleotide sequence comprises all of these Vκ gene segments. In an example, the first nucleotide sequence comprises one, more or all human V_K gene segments selected from the group consisting of V_K4-1*01, V_K2-28*01, V_K1-13*d01, V_K1- 12*01, V_K1-12*02, V_K3-20*01,V_K1-17*01,V_K1-39*01, V_K3-11*01, V_K1-16*01 and V_K1- 9*d01. For example, the nucleotide sequence comprises all of these Vκ gene segments. In one embodiment, the first nucleotide sequence comprises human gene segments VH3-23, JH2, VK4-1 and/or J_K2. In a further embodiment, the first nucleotide sequence comprises human gene segments VH3-7, JH6, VK2-28 and/or JK4. In a further embodiment, the first nucleotide sequence comprises human gene segments VH7-4-1, JH6, VK2-28 and/or JK4. The first nucleotide sequence further comprises D3-16. In a further embodiment, the first nucleotide sequence comprises human gene segments VH4-4-1, JH6, VK1-39 and/or JK4. The first nucleotide sequence further comprises D3-10. In a further embodiment, the first nucleotide sequence comprises human gene segments VH1-3, JH6, V_K1-12 and/or JK4. The first nucleotide sequence further comprises D3-10. In a further embodiment, the first nucleotide sequence comprises human gene segments VH3-13, JH6, VK1-12 and/or JK4. The first nucleotide sequence further comprises D3-9. In a further embodiment, the first nucleotide sequence comprises human gene segments VH4-4, JH6, VK1-39 and/or JK4. The first nucleotide sequence further comprises D3-10. In a further embodiment, the first nucleotide sequence comprises human gene segments VH3-13, JH6, VK3-20 and/or JK4. The first nucleotide sequence further comprises D3-10. In a further embodiment, the first nucleotide sequence comprises human gene segments VH3-23, JH6, VK1-17 and/or JK4. The first nucleotide sequence further comprises D3-22. In a further embodiment, the first nucleotide sequence comprises human gene segments VH3-7, JH6, VK1-39 and/or JK4. The first nucleotide sequence further comprises D3-9. In a further embodiment, the first nucleotide sequence comprises human gene segments VH3-13, JH6, VK1-39 and/or JK4. The first nucleotide sequence further comprises D3-10. In a further embodiment, the first nucleotide sequence comprises human gene segments VH3-13, JH6, VK3-11 and/or JK4. The first nucleotide sequence further comprises D3-10. In a further embodiment, the first nucleotide sequence comprises human gene segments VH4-4, JH6, VK1-16 and/or JK4. The first nucleotide sequence further comprises D3-9. In a further embodiment, the first nucleotide sequence comprises human gene segments VH3-20*d01, JH6, VK1-9 and/or JK4. The first nucleotide sequence further comprises D3-10. In a further embodiment, the first nucleotide sequence comprises human gene segment VH3-23. Additionally or alternatively, heavy chain variable domains of the antibody of the invention are encoded by (i) human VH3-23, D and JH segments. In a further embodiment, the first nucleotide sequence comprises human gene segment VH3-9. Additionally or alternatively, heavy chain variable domains of the antibody of the invention are encoded by (i) human VH3-9, D and JH segments. In a further embodiment, the first nucleotide sequence comprises human gene segment V_K1 -12 or V_K1D-12. Additionally or alternatively, light chain variable domains of the antibody of the invention are encoded by (i) human V_K1-12 or V_K1-12 and J_K segments. In a further embodiment, the first nucleotide sequence comprises human gene segment V_K2- 28. Additionally or alternatively, light chain variable domains of the antibody of the invention are encoded by (i) human VK2-28 and JK segments. In a further embodiment, the first nucleotide sequence comprises human gene segment V_K4-1. Additionally or alternatively, light chain variable domains of the antibody of the invention are encoded by (i) human V_K4-1 and J_K segments. In a further embodiment, the first nucleotide sequence comprises human combination of gene segments described above with JH6 in place of JH6. In one embodiment, the first nucleotide sequence comprises human gene segments VH3-23*04, JH2*01, VK4-1*01 and/or J_K2*01. In a further embodiment, the first nucleotide sequence comprises human gene segments VH3-7*01, JH6*02, VK2-28*01 and/or JK4*01. In a further embodiment, the first nucleotide sequence comprises human gene segments VH7-4- 1*01, JH6*02, VK2-28*01 and/or JK4*01. The first nucleotide sequence further comprises D3-16*02. In a further embodiment, the first nucleotide sequence comprises human gene segments VH4-4-1*02, JH6*02, VK1D-39*01 and/or JK4*01. The first nucleotide sequence further comprises D3-10*01. In a further embodiment, the first nucleotide sequence comprises human gene segments VH1-3*01, JH6*02, V_K1-12*01 and/or JK4*01. The first nucleotide sequence further comprises D3-10*01. In a further embodiment, the first nucleotide sequence comprises human gene segments VH3-13*01, JH6*02, VK1D-12*02 and/or JK4*01. The first nucleotide sequence further comprises D3-9*01. In a further embodiment, the first nucleotide sequence comprises human gene segments VH4-4*02, JH6*02, VK1D-39*01 and/or JK4*01. The first nucleotide sequence further comprises D3- 10*01. In a further embodiment, the first nucleotide sequence comprises human gene segments VH3-13*01, JH6*02, VK3-20*01 and/or JK4*01. The first nucleotide sequence further comprises D3-10*01. In a further embodiment, the first nucleotide sequence comprises human gene segments VH3-23*04, JH6*02, VK1 -17*01 and/or JK4*01. The first nucleotide sequence further comprises D3-22*01. In a further embodiment, the first nucleotide sequence comprises human gene segments VH3-7*01, JH6*02, VK1D-39*01 and/or JK4*01. The first nucleotide sequence further comprises D3-9*01. In a further embodiment, the first nucleotide sequence comprises human gene segments VH3-13*01, JH6*02, VK1D-39*01 and/or JK4*01. The first nucleotide sequence further comprises D3- 10*01. In a further embodiment, the first nucleotide sequence comprises human gene segments VH3-13*01, JH6*02, VK3-11*01 and/or J K4*01. The first nucleotide sequence further comprises D3-10*01. In a further embodiment, the first nucleotide sequence comprises human gene segments VH4-4*02, JH6*02, VK1 D-16*01 and/or JK4*01. The first nucleotide sequence further comprises D3-9*01. In a further embodiment, the first nucleotide sequence comprises human gene segments VH3-20*d01, JH6*02, VK1-9*d01 and/or JK4*01. The first nucleotide sequence further comprises D3-10*01. In a further embodiment, the first nucleotide sequence comprises human gene segment VH3-23*04. Additionally or alternatively, heavy chain variable domains of the antibody of the invention are encoded by (i) human VH3-23*04, D and JH segments. In a further embodiment, the first nucleotide sequence comprises human gene segment VH3-9*01. Additionally or alternatively, heavy chain variable domains of the antibody of the invention are encoded by (i) human VH3-9*01, D and JH segments. In a further embodiment, the first nucleotide sequence comprises human gene segment V_K1 -12*02 or V_K1D-12*02. Additionally or alternatively, light chain variable domains of the antibody of the invention are encoded by (i) human V_K1-12*02 or V_K1D- 12*02 and J_K segments. In a further embodiment, the first nucleotide sequence comprises human gene segment V_K2-28*01. Additionally or alternatively, light chain variable domains of the antibody of the invention are encoded by (i) human V_K2-28*01 and J_K segments. In a further embodiment, the first nucleotide sequence comprises human gene segment V_K4-1*01. Additionally or alternatively, light chain variable domains of the antibody of the invention are encoded by (i) human V_K4-1*01 and J_K segments. In a further embodiment, the first nucleotide sequence comprises human combination of gene segments described above with JH6*01 in place of JH6*02. In all embodiments described herein, the heavy chain V and J region can be recombined with each other and with a D region defined herein to form a heavy chain variable domain. In addition, the light chain V and J regions can be recombined to form a light chain variable domain. The invention extends to an antibody or antigen binding fragment comprising human variable domains produced or derived from recombination of any of the above combinations of gene segments by a cell or vertebrate of the invention. In an example, said heavy chain variable region of the locus comprises at least 5, 10, 15 or all human VH gene segments of the group consisting of IGHV1-3, IGHV1-8, IGHV1-18, IGHV1-46, IGHV3-7, IGHV3-9, IGHV3-11, IGHV3-15, IGHV3-20, IGHV3-21, IGHV3-23, IGHV3-30, IGHV3-33, IGHV3-48, IGHV3-53, IGHV4-4, IGHV4-31, IGHV4-34, IGHV4- 39, IGHV4-59, IGHV4-61, IGHV5-51, IGHV6-1 and IGHV7-4-1. In an example, the locus of the invention encodes a light chain, wherein the light chain pairs with or is capable of pairing with at least 5, 10, 15 or all human VH gene segments of the group consisting of IGHV1-3, IGHV1-8, IGHV1-18, IGHV1-46, IGHV3-7, IGHV3-9, IGHV3-11, IGHV3-15, IGHV3-20, IGHV3-21, IGHV3-23, IGHV3-30, IGHV3-33, IGHV3- 48, IGHV3-53, IGHV4-4, IGHV4-31, IGHV4-34, IGHV4-39, IGHV4-59, IGHV4-61, IGHV5-51, IGHV6-1 and IGHV7-4-1. Thus, the light chain can act as a common light chain for pairing with antibody heavy chains. In an example, the light chain pairs with or is capable of pairing with human VH gene segments IGHV1-3, IGHV1-8, IGHV1-18, IGHV1-46, IGHV3-7, IGHV3-9, IGHV3-11, IGHV3-15, IGHV3-20, IGHV3-21, IGHV3-23, IGHV3-30, IGHV3-33, IGHV3-48, IGHV3- 53, IGHV4-4, IGHV4-31, IGHV4-34, IGHV4-39, IGHV4-59, IGHV4-61, IGHV5-51, IGHV6-1 and IGHV7-4-1 in a vertebrate of the invention. In an example, said heavy chain variable region of the locus comprises at least 5, 10, 15 or all human VH gene segments of the group consisting of IGHV1-3*01, IGHV1-8*01, IGHV1- 18*01, IGHVl-46*03, IGHV3-7*01, IGHV3-9*01, IGHV3-11*01, IGHV3-15*01, IGHV3- 20*d01, IGHV3-21*03, IGHV3-23*04, IGHV3-30*18, IGHV3-33*01, IGHV3-48*02, IGHV3-53*01, IGHV4-4*02, IGHV4-31*03, IGHV4-34*01, IGHV4-39*01, IGHV4-59*01, IGHV4-61*01, IGHV5-51*01, IGHV6-1*01 and IGHV7-4-l*01. In an example, the locus of the invention encodes a light chain, wherein the light chain pairs with or is capable of pairing with at least 5, 10, 15 or all human VH gene segments of the group consisting of IGHV1-3*01, IGHV1-8*01, IGHV1-18*01, IGHVl-46*03, IGHV3-7*01, IGHV3-9*01, IGHV3-11*01, IGHV3-15*01, IGHV3-20*d01, IGHV3-21*03, IGHV3-23*04, IGHV3-30*18, IGHV3-33*01, IGHV3-48*02, IGHV3-53*01, IGHV4-4*02, IGHV4-31*03, IGHV4-34*01, IGHV4-39*01, IGHV4-59*01, IGHV4-61*01, IGHV5-51*01, IGHV6-1*01 and IGHV7-4-l*01. In an example, the light chain pairs with or is capable of pairing with human VH gene segments IGHV1-3*01, IGHV1-8*01, IGHV1-18*01, IGHVl-46*03, IGHV3-7*01, IGHV3- 9*01, IGHV3-11*01, IGHV3-15*01, IGHV3-20*d01, IGHV3-21*03, IGHV3-23*04, IGHV3-30*18, IGHV3-33*01, IGHV3- 48*02, IGHV3-53*01, IGHV4-4*02, IGHV4-31*03, IGHV4-34*01, IGHV4-39*01, IGHV4-59*01, IGHV4-61*01, IGHV5-51*01, IGHV6-1*01 and IGHV7-4-l*01 in a vertebrate of the invention. For example, the rearranged variable region is a rearrangement of (a) human IGLV3-21 and IGLJ3 (eg, IGLV3-21*01 and IGLJ3*02); (b) human IGK1-39 and IGKJ1 or 5; (c) human IGK3-20 and IGKJ1 or 5; or (d) a human VpreB and Jλ5. These variable regions can act as promiscuous regions capable of pairing with several different human variable regions. For example, T-cells of the vertebrate comprise the locus and endogenous TCR chain production is inactivated. In an embodiment, the vertebrate comprises B- or T-cells, wherein each cell comprises a said locus and expresses a single type of said polypeptide. In an example, the locus comprises (i) the functional TCRBV, D and J gene segments of a human TCRβ locus from TCRBV19 to TCRBJ1-1 inclusive, and optionally up to TCRBJl-6; or (ii) the functional TCRAV and J gene segments of a human TCRct locus from TCRAV24 to TCRAJ61 inclusive, and optionally up to TCRAJ1. As the skilled addressee will know, functional gene segments are denoted as green boxes in the locus representations shown in the IMGT Repertoire database. In an example, a locus of the invention comprises human TCRBV19, TCRBV20-1, TCRBV24-1, TCRBV25-1, TCRBV27, TCRBV28 and TCRBV29-1. For example, the TCRBV gene segments are at an Ig locus, eg, an IgH locus, eg, an endogenous IgH locus of the vertebrate or cell. In an example, the locus further comprises one or TCRBD gene segments and TCRBJl-1, TCRBJl-2, TCRBJl-3, TCRBJl-4, TCRBJl-5 and TCRBJl-6. In an example, the TCR V gene segment is selected from the group consisting of TRBV 19*01, 20- 1*02, 24-1*01, 25-1*01, 27*01, 28*01 and 29-01*01. For example, the TCR V is 20-1 (eg, 20-1*02). For example, the TCR V is 27 (eg, 27*01). In an example, the TCR J gene segment is selected from the group consisting of TRBJ 1-1*01, 1-2*01, 1-3*01, 1-4*01, 1-5*01 and 1- 6*01. For example, the TCR J is TCRBJ 1-5 (eg, 1-5*01). In an example, a) the one or more V gene segments are TCRAV segments and the one or more J gene segments are TCRAJ gene segments; b) the one or more V gene segments are TCRBV segments and the one or more J gene segments are TCRBJ gene segments; c) the one or more V gene segments are TCRCV segments and the one or more J gene segments are TCRCJ gene segments; or d) the one or more V gene segments are TCRDV segments and the one or more J gene segments are TCRDJ gene segments. In an example, alternatively to an unrearranged variable region, the locus of the invention comprises a rearranged TCR VJ (eg, VαJα or VγJγ) or VDJ (eg, VβDβJβ or VδDδJδ). The rearranged VJ or VDJ may be human or synthetic, for example. In an example, the vertebrate comprises such a rearranged TCR VJ operably linked upstream of an endogenous CL constant region (eg, at a mouse or rat endogenous kappa locus) and an unrearranged V-D-J region operably linked upstream of an endogenous IgH constant region (eg, at a mouse or rat endogenous IgH locus). In an example, the or a rearranged VDJ herein is the product of rearrangement of a V, D and J, wherein the V/J are selected from the group consisting of TCRBV27/TCRBJ1-5, TCRBV27/TCRBJ1-1, TCRBV20-1/TCRBJ1-5, TCRBV20-1/TCRBJ1-2, TCRBV20- 1/TCRBJ1-4, TCRBV29-1/TCRBJ1-5, TCRBV28/TCRBJ1-5, TCRBV20-1/TCRBJ1-1, TCRBV27/TCRBJ1-2 and TCRBV29-1/TCRBJ1-4. In an alternative, the group consists of TCRBV27*01/TCRBJl-5, TCRBV27*01/TCRBJ1-1, TCRBV20-l*02/TCRBJl-5, TCRBV20- l*02/TCRBJl-2, TCRBV20-l*01/TCRBJl-4, TCRBV29-l*02/TCRBJl-5, TCRBV28*01/TCRBJl-5, TCRBV20-1*02/TCRBJ1-1, TCRBV27*01/TCRBJl-2 and TCRBV29-l*01/TCRBJl-4. In an alternative, the group consists of TCRBV27*01/TCRBJ1- 5*01, TCRBV27*01/TCRBJ1-1*01, TCRBV20-1*02/TCRBJ1-5*01, TCRBV20- 1*02/TCRBJ1-2*01, TCRBV20-1*01/TCRBJ1-4*01, TCRBV29-1*02/TCRBJ1-5*01, TCRBV28*01/TCRBJ1-5*01, TCRBV20-1*02/TCRBJ1-1*01, TCRBV27*01/TCRBJ1-2*01 and TCRBV29-1*01/TCRBJ1-4*01. In an example, the or a rearranged VDJ herein is the product of rearrangement of a V, D and J, wherein the V/J is TCRBV27/TCRBJ1-5, eg, TCRBV27*01/TCRBJl-5 or TCRBV27*01/TCRBJ1-5*01. In an example, the vertebrate expresses a plurality of different rearranged TCR VDJ, wherein each VDJ is the product of rearrangement of a V, D and J, wherein the V/J are selected from the group consisting of TCRBV27/TCRBJ1-5, TCRBV27/TCRBJ1-1, TCRBV20-1/TCRBJ1- 5, TCRBV20-1/TCRBJ1-2, TCRBV20-1/TCRBJ1-4, TCRBV29-1/TCRBJ1-5, TCRBV28/TCRBJ1-5, TCRBV20-1/TCRBJ1-1, TCRBV27/TCRBJ1-2 and TCRBV29- 1/TCRBJ1-4. In an alternative, the group consists of TCRBV27*01/TCRBJl-5, TCRBV27*01/TCRBJ1-1, TCRBV20-l*02/TCRBJl-5, TCRBV20-l*02/TCRBJl-2, TCRBV20-l*01/TCRBJl-4, TCRBV29-l*02/TCRBJl-5, TCRBV28*01/TCRBJl-5, TCRBV20-1*02/TCRBJ1-1, TCRBV27*01/TCRBJl-2 and TCRBV29-l*01/TCRBJl-4. In an alternative, the group consists of TCRBV27*01/TCRBJ1-5*01, TCRBV27*01/TCRBJ1- 1*01, TCRBV20-1*02/TCRBJ1-5*01, TCRBV20-1*02/TCRBJ1-2*01, TCRBV20- 1*01/TCRBJ1-4*01, TCRBV29-1*02/TCRBJ1-5*01, TCRBV28*01/TCRBJ1-5*01, TCRBV20-1*02/TCRBJ1-1*01, TCRBV27*01/TCRBJ1-2*01 and TCRBV29- 1*01/TCRBJ1-4*01. In an example, the plurality comprises one or more rearranged VDJ, wherein each VDJ is the product of rearrangement of a V, D and J, wherein the V/J is TCRBV27/TCRBJ1-5, eg, TCRBV27*01/TCRBJl-5 or TCRBV27*01/TCRBJ1-5*01. In an example, the cell expresses a rearranged TCR VDJ which is the product of rearrangement of a V, D and J, wherein the V/J are selected from the group consisting of TCRBV27/TCRBJ1-5, TCRBV27/TCRBJ1-1, TCRBV20-1/TCRBJ1-5, TCRBV20-1/TCRBJ1-2, TCRBV20- 1/TCRBJ1-4, TCRBV29-l/TCRBJl-5, TCRBV28/TCRBJ1-5, TCRBV20-1/TCRBJ1-1, TCRBV27/TCRBJ1-2 and TCRBV29-1/TCRBJ1-4. In an alternative, the group consists of TCRBV27*01/TCRBJl-5, TCRBV27*01/TCRBJ1-1, TCRBV20-l*02/TCRBJl-5, TCRBV20- l*02/TCRBJl-2, TCRBV20-l*01/TCRBJl-4, TCRBV29-l*02/TCRBJl-5, TCRBV28*01/TCRBJl-5, TCRBV20-1*02/TCRBJ1-1, TCRBV27*01/TCRBJl-2 and TCRBV29-l*01/TCRBJl-4. In an alternative, the group consists of TCRBV27*01/TCRBJ1- 5*01, TCRBV27*01/TCRBJ1-1*01, TCRBV20-1*02/TCRBJ1-5*01, TCRBV20- 1*02/TCRBJ1-2*01, TCRBV20-1*01/TCRBJ1-4*01, TCRBV29-1*02/TCRBJ1-5*01, TCRBV28*01/TCRBJ1-5*01, TCRBV20-1*02/TCRBJ1-1*01, TCRBV27*01/TCRBJ1-2*01 and TCRBV29-1*01/TCRBJ1-4*01. In an example, the VDJ is the product of rearrangement of a V, D and J, wherein the V/J is TCRBV27/TCRBJ1-5, eg, TCRBV27*01/TCRBJl-5 or TCRBV27*01/TCRBJ1-5*01. In an example, the locus comprises a human, mouse or rat antibody locus intronic enhancer (eg, a Εμ or I EK enhancer) between the variable and constant regions and/or a human, mouse or rat antibody locus 3' enhancer operably linked downstream of said constant region.. For example, the enhancer is a mouse Εμ and the constant region is an antibody heavy chain constant region. For example, the enhancer is a mouse Eiκ and the constant region is an antibody kappa chain constant region. For example the 3' enhancer is a mouse antibody heavy chain locus 3' enhancer. For example the 3' enhancer is a mouse antibody kappa chain locus 3' enhancer. The constant region optionally comprises the endogenous antibody heavy chain locus Εμ and Sμ of the vertebrate, optionally wherein the constant region comprises the DNA sequence of the endogenous Εμ through to (and including) the Sμ of the vertebrate. The constant region optionally comprises the endogenous antibody heavy chain locus mu switch sequence (Sμ) of the vertebrate, the constant region comprising downstream of the Sμ a second switch sequence and a second C segment, wherein the constant region is capable of class-switch recombination (CSR) between the switches for isotype switching from the Sμ ΐο the second constant region gene segment and somatic hypermutation (SHM) of the TCR variable region. SHM is useful to produce a plurality of affinity matured TCR V domains, for example comprising antigen-binding affinities that are stronger than typically found for natural TCR binding sites and V domains. The constant region optionally comprises a) a first antibody C segment (eg, a Cμ) operably linked to a first switch sequence (eg, a 5μ); b) a second antibody C segment operably linked to a second switch sequence; c) wherein the constant region is capable of CSR between the switches for isotype switching from the first to the second C segment and SHM of the TCR variable region. Operable linkage in this respect will be clear to the skilled person as involving the usual recombination between switch sequences to effect CSR and isotype switching, as seen in IgH loci. In an example, the second C segment is a human or a mouse gamma C, eg, gamma-1, gamma-2, gamma-3 or gamma-4 C segment. In an example, the segment is a mouse gamma-1 C, eg, an endogenous C when the vertebrate is a mouse or rat. In an example, the segment is a human gamma-1 C. In an example, the genome of the vertebrate comprises an endogenous activation induced cytidine deaminase (AID) nucleotide sequence that is capable of expressing AID for SHM of the TCR variable region. The vertebrate genome may comprise a nucleotide sequence for expressing an endogenous RAG-1 and/or RAG-2. A particularly useful example is a vertebrate that expresses paired TCR V domains that provide an antigen binding site, wherein the V domains are encoded by loci of the invention (eg, after rearrangement of the variable region and SHM following exposure of the vertebrate to the antigen). There is provided, therefore, in an embodiment a vertebrate of the invention comprising a first locus and a second locus, wherein a) the TCR variable region of the first locus comprises one or more TCRAV segments and one or more TCRAJ gene segments and optionally the one or more antibody C gene segments are kappa C segments; and the TCR variable region of the second locus comprises one or more TCRBV segments, one or more TCRBD segments and one or more TCRBJ gene segments and optionally the one or more antibody C gene segments are heavy chain C segments, wherein the antigen binding site of each ligand comprises a TCR Vet domain and a TCR νβ domain and optionally paired antibody heavy and kappa C domains; or b) the TCR variable region of the first locus comprises one or more TCRGV segments and one or more TCRGJ gene segments and optionally the one or more antibody C gene segments are kappa C segments; and the TCR variable region of the second locus comprises one or more TCRDV segments, one or more TCRDD segments and one or more TCRDJ gene segments and optionally the one or more antibody C gene segments are heavy chain C segments, wherein the antigen binding site of each ligand comprises a TCR Vy domain and a TCR V6 domain and optionally paired antibody heavy and kappa C domains. Thus, such ligands expressed from the vertebrate are a useful source of paired VαVβ or VγVδ antigen binding sites, or a source of the V domains per se; and/or a source of nucleotide sequences encoding these. For example, the invention contemplates isolating or copying such a nucleotide sequence and inserting it into an expression vector (eg, harboured by a host cell, such as a CHO or HEK293 or other cell) for expression of the cognate V domain. By inserting such a nucleotide sequence encoding a TC Vα domain into the genome of the cell, and inserting a nucleotide sequence encoding a TCR νβ into the genome, the cell can express a VαVβ paired antigen binding site; and the cell can be grown into a cell line for expressing such a binding site. By inserting such a nucleotide sequence encoding a TCR Vγ domain into the genome of the cell, and inserting a nucleotide sequence encoding a TCR Vδ into the genome, the cell can express a VγVδ paired antigen binding site; and the cell can be grown into a cell line for expressing such a binding site. In an example, the V and J gene (and optional D) segments are human gene segments, optionally wherein the antibody C gene segments are human, rat or mouse gene segments. In an example, one or more or all of the V gene segments is synthetic, eg, each V is a mutated germline V gene segment. In an example, one or more or all of the D gene segments is synthetic, eg, each D is a mutated germline D gene segment. In an example, one or more or all of the J gene segments is synthetic, eg, each J is a mutated germline J gene segment. The or each variable region is optionally not at an endogenous antibody locus. For example, the locus (or one or all of the loci) is a product of random insertion into the vertebrate genome. For example, the locus has been targeted into the genome, eg, the locus is at a Rosa 26 locus. The vertebrate may be obtainable or obtained in a method by a) providing an embryonic stem cell of the vertebrate species (eg, mouse or rat); b) inserting DNA comprising said variable region gene segments into the ES cell genome in one or several steps to produce an ES cell product whose genome comprises the inserted variable region DNA operably linked upstream of the antibody constant region for expression of said polypeptides (ie, in a vertebrate developed from the cell or a progeny thereof); and c) developing said vertebrate from said product ES cell or a progeny thereof; d) wherein either e) the variable region DNA (eg, antibody heavy or TCR beta variable region DNA) is inserted into an endogenous antibody locus (eg, a heavy chain locus) of the genome and the constant region comprises one or more C gene segments of the endogenous locus, wherein the insertion produces an engineered locus that is capable of expressing said polypeptides and CS and SHM of the variable region; or f) the variable region DNA (eg, antibody heavy or TCR beta variable region DNA) is comprised by a transgene, wherein the transgene comprises said constant region (eg, comprising a CH gene segment), wherein the transgene is inserted into said genome to provide a transgene locus that is capable of expressing said polypeptides and CSR and SHM of the variable region. The step of inserting DNA in step (b) can be performed in one or multiple steps (depending, for example, upon the amount of DNA to be inserted) using standard techniques, eg, employing BACs and homologous recombination and/or site-specific recombination (eg, RMCE using cre-lox technology). The insertion may not concomitantly delete endogenous DNA or it may do so simultaneously or before or after the insertion. DNA insertion may be in several smaller parts using a plurality of ES cells (such as using standard techniques involving insertions into genomes of ES cells in a lineage). Optionally, it may be desirable to re-derive ES cells from mice or other vertebrates during the process, wherein the re-derived ES cells receive one or more further insertions of DNA. All of these techniques for building ES cell genomes by targeted insertion and developing mice, rats or other vertebrates from ES cells are conventional and known to the skilled person. The step of developing the vertebrate from the product ES cell can also be performed conventionally by inserting the ES cell into a blastocysts and implanting a pseudopregnant mother. Chimaera progeny can be made and crossed to produce progeny mice which are according to the invention (eg comprising a homozygous locus according to the invention). By providing exogenous V gene segments at an endogenous antibody locus, the endogenous control of the locus can be harnessed (eg, for proper functioning of the locus in a B-cell to express V domains). Optionally, in the method the insertion in step (d) of variable region DNA is an insertion (i) immediately 5' of the 5' -end of the intron of said endogenous antibody locus; or (ii) between said 5' end and the intronic enhancer (eg, Εμ) of the intron. The intron is the stretch of DNA naturally contiguous with and immediately 3' of the last (3'-most) antibody J segment in an antibody locus to and including the nucleotide naturally immediately 5' of the Cmu or CL. Optionally, (i) the engineered locus comprises less than the complete intronic sequence immediately 5' of said intronic enhancer found in wild-type vertebrates of said species; and/or (ii) the distance between the last inserted human J gene segment and said intronic enhancer is not >1 or 0.5 kb more (or no more) than the distance between the last antibody J gene segment and the enhancer found in wild-type vertebrates of said species; and/or (iii) the inserted DNA comprises a 3'-most TC J gene segment, wherein the segment is immediately 5' of a further nucleotide sequence, wherein the further sequence is intron sequence that is naturally contiguous (ie, in a wild-type respective TCR locus in the genome of a human or other species from which the inserted DNA is derived) with said TCR J segment and the further sequence is no more than 1 or 0.5 kb in length. In an example, the locus comprises complete intronic sequence immediately 5' of said intronic enhancer found in wild-type vertebrates of said species, but with the omission of up to the first (ie, 5'-most) contiguous lkb, 900bp, 800bp, 700bp, 600bp, 500bp, 400bp, 300bp, 200bp or l00bp of the intron found in said wild-type vertebrates. Thus, the locus comprises such a wild-type intronic sequence that is missing its first (5'-most) contiguous 1000-l00bps. The invention also contemplates a non-human vertebrate that is a progeny of the vertebrate developed in step (c) of the method, wherein the progeny vertebrate is according to the invention. In any example herein, the vertebrate may be incapable of antibody heavy chain and/or kappa chain variable region expression. The vertebrate may additionally or alternatively be incapable of antibody lambda chain expression. This may be achieved by deleting or disrupting one or more respective antibody loci or variable regions in the germline genome of the vertebrate (eg, by J region deletion, neo insertion into an endogenous V region and/or inversion of an endogenous V region). In an example, the vertebrate may be incapable of non- human vertebrate antibody heavy chain and/or kappa chain variable region expression. In alternative, the vertebrate is capable of expressing antibody heavy chains (eg, from a genomically-integrated transgene or an allele of an endogenous IgH locus, such as where the other allele is a locus according to the invention) and/or light chains (eg, from a genomically- integrated transgene or an allele of an endogenous IgL locus, such as where the other allele is a locus according to the invention). In alternative, the vertebrate is capable of expressing antibody heavy chains (eg, from a genomically-integrated transgene or an allele of an endogenous IgH locus, such as where the other allele is a locus according to the invention) and/or light chains (eg, from a genomically- integrated transgene or an allele of an endogenous IgL locus, such as where the other allele is a locus according to the invention). In an example, the germline genome of the vertebrate comprises one or more expressible ADAM6 nucleotide sequences (eg, mouse ADAM6a and/or ADAM6b, eg, wherein the vertebrate is a mouse), eg, in homozygous state. In an example, the vertebrate has wild-type fertility typical of wild-type vertebrates of the same species that comprise functional homozygous ADAM6 genes. In an example, the vertebrate is incapable of non-human vertebrate (i) TC Vβ domain and/or TCR Vα domain expression (eg, wherein EM1 is a human TCR Vβ domain and/or TCR Vα domain); (ii) TCR Vδ domain and/or TCR Vγ domain expression (eg, wherein EM1 is a human TCR Vδ domain and/or TCR Vγ domain); or (iii) TCR Vβ and TCR Vδ domain expression (eg, wherein EM1 is a human TCR Vβ and TCR Vγ). In an example wherein EM1 and/or EM2 is a TCR V domain, the vertebrate comprises antigen presenting cells comprising nucleic acid for surface expressing a peptide antigen receptor comprising a human MHC protein (eg, Class I or Class II M HC), wherein the vertebrate is capable of producing said plurality of multimers of the invention when the vertebrate is immunised with a peptide-MHC antigen (pMHC) comprising said human MHC protein. This is useful as the vertebrate expresses the MHC as self-protein, thereby focusing the immune response on the peptide antigen with which the vertebrate is immunised and which the TCR V is capable of recognising and binding. In an embodiment, the vertebrate additionally (when the MHC is class I MHC) or alternatively expresses human beta-2 microglobulin which is capable of forming peptide-presenting complex with the MHC in the vertebrate. In an embodiment, in these instances the vertebrate expresses human TCR Vβ domain and/or TCR Vα domains (which are EM1 and/or EM2) that form a binding site for said pMHC. Thus, for example the vertebrate comprises antigen presenting cells that further comprise nucleic acid for surface expressing human beta-2 microglobulin complexed with the MHC protein, wherein the M HC protein is a human class I MHC protein, eg, HLA-A2. This is further useful for focussing the immune response just to the peptides, as the antigen receptor components will be seen as self in the vertebrate. An aspect of the invention provides a non-human vertebrate embryo which is capable of developing into a vertebrate of the invention. In an embodiment the embryo or vertebrate of the invention is male. In an embodiment the embryo or vertebrate of the invention is female. In an embodiment the vertebrate of the invention is an adult. In an embodiment the vertebrate of the invention is an infant. In an embodiment the embryo or vertebrate of the invention is a chimaera of two or more genomes of said non-human vertebrate species (eg, two mouse strains). An aspect of the invention provides an isolated ES cell, iPS cell, immune cell (eg, NK cell or TIL), B-cell; thymus cell (eg, T-cell) or tissue; spleen cell or tissue; or bone marrow cell or tissue obtainable or obtained from a vertebrate of the invention, eg, in a sterile container. Another aspect provides a plurality (eg, at least 10, 100, 1000, 10⁴, 10⁵, 10^s, 10⁷, 10⁸, 10⁹, 10¹⁰ or 10¹¹) of said immune, B-, thymus, T-, spleen or bone marrow cells, eg, in a sterile container. The container may be an IV bag, syringe, test tube, flask or petri dish. For example, the rearranged variable region DNA may encode a variable region that is capable of binding to any antigen disclosed herein. For example, any variable domain or effector moiety is capable of binding to any antigen disclosed herein. For example, EM1 is capable of binding to any antigen disclosed herein. For example, EM2 is capable of binding to any antigen disclosed herein. Exemplary antigens are: ABCF1; ACVR1; ACVR1B; ACVR2; ACVR2B; ACVRL1; ADORA2A; Aggrecan; AGR2; AICDA; AWI; AIG1; AKAP1; AKAP2; AIYIH; AMHR2; ANGPT1; ANGPT2; ANGPTL3; ANGPTL4; ANPEP; APC; APOC1; AR; AZGP1 (zinc-a-glycoprotein); B7.1; B7.2; BAD; BAFF; BAG1; BAI1; BCL2; BCL6; BDNF; BLNK; BLRl (MDR15); BlyS; BM Pl; BMP2; BMP3B (GDFIO); BMP4; BMP6; BM P8; BMPRIA; BMPRIB; BM PR2; BPAG1 (plectin); BRCA1; CI9orflO (IL27w); C3; C4A; C5; C5R1; CANT1; CASP1; CASP4; CAV1; CCBP2 (D6 / JAB61); CCL1 (1-309); CCL11 (eotaxin); CCL13 (MCP-4); CCL15 (MIP-id); CCL16 (HCC-4); CCL17 (TARC); CCL18 (PARC); CCL19 (M IP-3b); CCL2 (MCP-1); MCAF; CCL20 (MIP-3a); CCL21 (MIP-2); SLC; exodus-2; CCL22 (MDC / STC-1); CCL23 (M PIF- 1); CCL24 (MPIF-2 I eotaxin-2); CCL25 (TECK); CCL26 (eotaxin-3); CCL27 (CTACK /ILC) ; CCL28; CCL3 (MIP-la); CCL4 (M IP-lb); CCL5 (RANTES); CCL7 (MCP-3); CCL8 (mcp-2); CCNA1; CCNA2; CCND1; CCNE1; CCNE2; CCR1 (CKR1 / HM145); CCR2 (mcp-1RB / RA);CCR3 (CKR3 / CMKBR3); CCR4; CCR5 (CM KBR5 / ChemR13); CCR6 (CMKBR6 / CKR-L3 / STRL22 / DRY6); CCR7 (CKR7 / EBI1); CCR8 (CM KBR8 / TER1 / CKR-L1); CCR9 (GPR-9-6); CCRL1 (VSHK1); CCRL2 (L-CCR); CD164; CD19; CD1C; CD20; CD200; CD-22; CD24; CD28; CD3; CD37; CD38; CD3E; CD3G; CD3Z; CD4; CD40; CD40L; CD44; CD45RB; CD52; CD69; CD72; CD74; CD79A; CD79B; CD8; CD80; CD81; CD83; CD86; CDH1 (E-cadherin); CDH10; CDH12; CDH13; CDH18; CDH19; CDH20; CDH5; CDH7; CDH8; CDH9; CDK2; CDK3; CDK4; CDK5; CDK6; CDK7; CDK9; CDKN1A (p2IWapl/Cipl); CDKN1B (p27Kipl); CDKNIC; CDKN2A (pl6INK4a); CDKN2B; CDKN2C; CDKN3; CEBPB; CER1; CHGA; CHGB; Chitinase; CHST10; CKLFSF2; CKLFSF3; CKLFSF4; CKLFSF5; CKLFSF6; CKLFSF7; CKLFSF8; CLDN3; CLDN7 (claudin-7); CLN3; CLU (clusterin); CMKLR1; CMKOR1 (RDC1); CNR1; COL18A1; COL1A1; COL4A3; COL6A1; CR2; CRP; CSFl (M-CSF); CSF2 (GM-CSF); CSF3 (GCSF); CTLA4; CTNNBl (b-catenin); CTSB (cathepsin B); CX3CL1 (SCYDi) ; CX3CR1 (V28); CXCL1 (GROl); CXCLIO (IP-10); CXCL11 (l-TAC / IP-9); CXCL12 (SDF1); CXCL13; CXCL14; CXCL16; CXCL2 (GR02); CXCL3 (GR03); CXCL5 (ENA-78 I LIX); CXCL6 (GCP-2); CXCL9 (MIG); CXCR3 (GPR9/CKR-L2); CXCR4; CXCR6 (TYMSTR ISTRL33 I Bonzo); CYB5; CYC1; CYSLTR1; DAB2IP; DES; DKFZp451J0118; DNCL1; DPP4; E2F1; ECGF1; EDG1; EFNAI; EFNA3; EFNB2; EGF; EGFR; ELAC2; ENG; EN01; EN02; EN03; EPHB4; EPO; ERBB2 (Her-2); EREG; ERK8; ESR1; ESR2; F3 (TF); FADD; FasL; FASN; FCER1A; FCER2; FCGR3A; FGF; FGF1 (aFGF); FGF10; FGF11; FGF12; FGF12B; FGF13; FGF14; FGF16; FGF17; FGF18; FGF19; FGF2 (bFGF); FGF20; FGF21; FGF22; FGF23; FGF3 (int-2); FGF4 (HST); FGF5; FGF6 (HST-2); FGF7 (KGF); FGF8; FGF9; FGFR3; FIGF (VEGFD); FILl (EPSILON); FILl (ZETA); FU12584; FU25530; FLRTl (fibronectin); FLTl; FOS; FOSLl (FRA-I); FY (DARC); GABRP (GABAa); GAGEB1; GAGEC1; GALNAC4S-65T; GATA3; GDF5; GFI1; GGT1; GM-CSF; GNAS1; GNRHl; GPR2 (CCRIO); GPR31; GPR44; GPR81 (FKSG80); GRCCIO (CIO); GRP; GSN (Gelsolin); GSTPl; HAVCR2; HDAC4; EDAC5; HDAC7A; HDAC9; HGF; HIF1A; HIP1; histamine and histamine receptors; HLA-A; HLA-DRA; HM74; HMOX1; HUMCYT2A; ICEBERG; ICOSL; 1D2; IFN-a; IFNA1; IFNA2; IFNA4; IFNA5; IFNA6; IFNA7; IFNB1; IFNgamma; TFNW1; IGBP1; IGF1; IGF1R; IGF2; IGFBP2; IGFBP3; IGFBP6; IL-1; IL10; IL10RA; IL10RB; IL11; IL11RA; IL-12; IL12A; IL12B; IL12RB1; IL12RB2; 1L13; IL13RA1; IL13RA2; 1L14; 1L15; IL15RA; IL16; 1L17; IL17B; IL17C; IL17R; 1L18; IL18BP; IL18R1; IL18RAP; 1L19; ILIA; IL1B; IL1F10; IL1F5; IL1F6; IL1F7; IL1F8; IL1F9; IL1HY1; IL1R1; IL1R2; IL1RAP; IL1RAPL1; IL1RAPL2; IL1RL1;IL1RL2 IL1RN; 1L2; 1L20; IL20RA; IL21R; 1L22; 1L22R; 1L22RA2; 1L23; 1L24; 1L25; 1L26; 1L27; 1L28A; 1L28B; 1L29; IL2RA; IL2RB; IL2RG; 1L3; 1L30; IL3RA; 1L4; IL4R; 1L5; IL5RA; 1L6; IL6R; IL6ST (glycoprotein 130); 1L7; TL7R; 1L8; IL8RA; IL8RB; IL8RB; 1L9; IL9R; ILK; INHA; INHBA; INSL3; INSL4; IRAKI; IRAK2; ITGA1; ITGA2; 1TGA3; ITGA6 (a6 integrin); ITGAV; ITGB3; ITGB4 (b 4 integrin); JAG1; JAK1; JAK3; JUN; K6HF; KAI1; KDR; MTLG; KLF5 (GC Box BP); KLF6; KLK10; KLK12; KLK13; KLK14; KLK15; KLK3; KLK4; KLK5; KLK6; KLK9; KRT1; KRT19 (Keratin 19); KRT2A; KRTHB6 (hair-specific type II keratin); LAMA5; LEP (leptin); Lingo-p75; Lingo-Troy; LPS; LTA (TNF-b); LTB; LTB4R (GPR16); LTB4R2; LTBR; MACMARCKS; MAG or Omgp; MAP2K7 (c-Jun); MDK; M IB1; midkine; M IF; M IP-2; MK167 (Ki-67); MMP2; M MP9; MS4A1; MSMB; MT3 (metallothionectin-ifi); MTSS 1; M UC 1 (mucin); MYC; MYD88; NCK2; neurocan; NFKB 1; NFKB2; NGFB (NGF); NGFR; NgR-Lingo; NgR-Nogo66 (Nogo); NgR-p75; NgR-Troy; NM E1 (NM23A); NOX5; NPPB; NROB1; NROB2; NR1D1; NR1D2; NR1H2; NR1H3; NR1H4; NR1I2; NR1I3; NR2C1; NR2C2; NR2E1; NR2E3; NR2F1; NR2F2; NR2F6; NR3C1; NR3C2; NR4A1; NR4A2; NR4A3; NR5A1; NR5A2; NR6A1; NRP1; NRP2; NT5E; NTN4; ODZ1; OPRD1; P2RX7; PAP; PARTI; PATE; PAWR; PCA3; PCNA; PDGFA; PDGFB; PECAM1; PF4 (CXCL4); PGF; PGR; phosphacan; PIAS2; PIK3CG; PLAU (uPA); PLG; PLXDC1; PPBP (CXCL7); PPID; PR1; PRKCQ; PRKD1; PRL; PROC; PROK2; PSAP; PSCA; PTAFR; PTEN; PTGS2 (COX-2); PTN; RAC2 (p2IRac2); RARB; RGS1; RGS13; RGS3; RNF110 (ZNF144); ROB02; S100A2; SCGB1D2 (lipophilin B); SCGB2A1 (mammaglobin 2); SCGB2A2 (mammaglobin 1); SCYE1 (endothelial Monocyte-activating cytokine); SDF2; SERPINA1; SERPINIA3; SERPINB5 (maspin); SERPINE1 (PAT-i); SERPINF1; SHBG; SLA2; SLC2A2; SLC33A1; SLC43A1; SLIT2; SPPl; SPRRIB (Spri); ST6GAL1; STABl; STAT6; STEAP; STEAP2; TB4R2; TBX21; TCPIO; TDGF1; TEK; TGFA; TGFB1; TGFB1I1; TGFB2; TGFB3; TGFBI; TGFBR1; TGFBR2; TGFBR3; TH1L; THBS1(thrombospondin-1); THBS2; THBS4; THPO; TIE (Tie-i); T]MP3; tissue factor; TLRIO; TLR2; TLR3; TLR4; TLR5; TLR6; TLR7; TLR8; TLR9; TNF; TNF-a (also referred to herein as TNF alpha or TNFα); TNFAIP2 (B94); TNFAIP3; TNFRSF11A; TNFRSF1A; TNFRSF1B; TNFRSF21; TNFRSF5; TNFRSF6 (Fas); TNFRSF7; TNFRSF8; TNFRSF9; TNFSFIO (TRAIL); TNFSF11 (TRANCE); TNFSF12 (AP03L); TNFSF13 (April); TNFSF13B; TNFSF14 (HVEM-L); TNFSF15 (VEGI); TNFSF18; TNFSF4 (0X40 ligand); TNFSF5 (CD40 ligand); TNFSF6 (FasL); TNFSF7 (CD27 ligand); TNFSF8 (CD30 ligand); TNFSF9 (4-lBB ligand); TOLLIP; Toll- like receptors; TOP2A (topoisomerase lia); TP53; TPM 1; TPM2; TRADD; TRAF1; TRAF2; TRAF3; TRAF4; TRAF5; TRAF6; TREM 1; TREM2; TRPC6; TSLP; TWEAK; VEGF; VEGFB; VEGFC; versican; VHL C5; VLA-4; XCL1 (lymphotactin); XCL2 (SCM-lb); XCR1 (GPR5 / CCXCR1); YY1; and ZFPM2. For example, the antigen is CD3. For example, the antigen is a Death Receptor, eg, DR5. For example the antigen is TNF alpha. For example, the antigen is VEGF. For example, the antigen is a coronavirus antigen, eg, a SARS-CoV-2 antigen, such as spike. For example, the antigen is BCMA. For example, the antigen is TACI. For example, the antigen is CD38. For example, the antigen is selected from the group consisting of EpCAM and CD3; CD19 and CD3; VEGF and VEGFR2; VEGF and EGFR; CD138 and CD20; CD138 and CD40; CD20 and CD3; CD38 and CD138; CD38 and CD20; CD38 and CD40; CD40 and CD20; CD19 and CD20; CD-8 and IL-6; PDL-1 and CTLA-4; CTLA-4 and BTN02; CSPGs and RGM A; IGF1 and IGF2; IGF1 and/or 2 and Erb2B; IL-12 and IL-18; IL-12 and TWEAK; IL-13 and ADAM8; IL-13 and CL25; IL-13 and IL-lbeta; IL-13 and IL-25; IL-13 and IL-4; IL-13 and IL-5; IL-13 and IL-9; IL-13 and LHR agonist; IL-13 and MDC; IL-13 and MIF; IL-13 and PED2; IL-13 and SPRR2a; IL-13 and SPRR2b; IL-13 and TARC; IL-13 and TGF-beta; IL-1 alpha and IL-1 beta; MAG and RGM A; NgR and RGM A; NogoA and RGM A; OMGp and RGM A; RGM A and RGM B; Te38 and TNF alpha; TNF alpha and IL-12; TNF alpha and IL-12p40; TNF alpha and IL-13; TNF alpha and IL-15; TNF alpha and IL-17; TNF alpha and IL-18; TNF alpha and IL-1 beta; TNF alpha and IL-23; TNF alpha and M IF; TNF alpha and PEG2; TNF alpha and PGE4; TNF alpha and VEGF; and VEGFR and EGFR; TNF alpha and RANK ligand; TNF alpha and Blys; TNF alpha and GP130; TNF alpha and CD-22; and TNF alpha and CTLA-4. For example, one of EM1 and EM2 is capable of binding to an antigen that is selected from the group consisting of CD3; CD16; CD32; CD64; and CD89; and the other of EM1 and EM2 is capable of binding to an antigen that is selected from the group consisting of EGFR; VEGF; IGF-1R; Her2; c-Met (aka HGF); HER3; CEA; CD33; CD79a; CD19; PSA; EpCAM; CD66; CD30; HAS; PSMA; GD2; ANG2; IL-4; IL-13; VEGFR2; and VEGFR3. In an example, EM1 or EM2 is capable of binding to an antigen selected from the group consisting of human IL-1A, IL-1β, IL-1RN, IL-6, BLys, APRIL, activin A, TNF alpha, a BMP, BMP2, BMP7, BMP9, BMP10, GDF8, GDF11, RANKL, TRAIL, VEGFA, VEGFB or PGF; optionally wherein the multimer comprises a cytokine amino acid sequence (eg, C- terminal to a TD), such as IL-2 or an IL2-peptide; and the multimer is for treating or preventing a cancer in a human subject. In an example the polypeptide comprises a cytokine amino acid sequence (eg, C-terminal to SAM), such as IL-2 or an IL2-peptide; and the multimer is for treating or preventing a cancer in a human subject. In an example, EM1 is a TCR variable domain that binds to a pMHC. In an example EM1 is capable of specifically binding to the antigen. In an example EM2 is capable of specifically binding to the antigen. By the term "specifically binds" and similar terms as used herein, eg, with respect to an effector moiety, is meant a moiety which recognises a specific antigen with a binding affinity of 1mM or less as determined by SPR. Binding of an effector moiety herein may be binding with an affinity determined by SPR. Binding of an effector moiety comprised by a polypeptide herein may be binding with an affinity determined by SPR. Binding of a multimer herein may be binding with an affinity determined by SPR. Target (antigen or pHMC) binding ability, specificity and affinity (KD (also termed Kd), K_off and/or K_on) can be determined by any routine method in the art, eg, by surface plasmon resonance (SPR). The term “KD”, as used herein, is intended to refer to the equilibrium dissociation constant of a particular binding site/ligand, receptor/ligand or antibody/antigen interaction. In one embodiment, the surface plasmon resonance (SPR) is carried out at 25° C. In another embodiment, the SPR is carried out at 37° C. In one embodiment, the SPR is carried out at physiological pH, such as about pH7 or at pH7.6 (eg, using Hepes buffered saline at pH7.6 (also referred to as HBS-EP)). In one embodiment, the SPR is carried out at a physiological salt level, eg, 150 mM NaCl. In one embodiment, the SPR is carried out at a detergent level of no greater than 0.05% by volume, eg, in the presence of P20 (polysorbate 20; eg, Tween-20™) at 0.05% and EDTA at 3 mM. In one example, the SPR is carried out at 25° C. or 37° C. in a buffer at pH7.6, 150 mM NaCl, 0.05% detergent (eg, P20) and 3 mM EDTA. The buffer can contain 10 mM Hepes. In one example, the SPR is carried out at 25° C. or 37° C. in HBS-EP. HBS-EP is available from Teknova Inc (California; catalogue number H8022). In an example, the affinity (eg, of a VH/VL binding site) is determined using SPR by using any standard SPR apparatus, such as by Biacore™ or using the ProteOn XPR36™ (Bio-Rad®). The binding data can be fitted to 1:1 model inherent using standard techniques, eg, using a model inherent to the ProteOn XPR36™ analysis software. Alternatively, as is known, binding may be determined by an ELISA assay, such as by determining OD₄₅₀, for example in an ELISA assay. Optionally, binding or binding strength is determined by ELISA, eg, by determining OD₄₅₀. An ELISA herein may be carried out at room temperature and pressure (rtp), or preferably at 20 or 25 degrees centigrade and 1 atmosphere. Optionally, the multimer binds to cognate antigen with an OD₄₅₀ from 1 to 3 (such as from 1 to 2 or from 2 to 3) in an ELISA assay in which the antigen is at a concentration of 1 nM in the assay. Binding of the multimer with said OD₄₅₀ indicates that the multimer is useful for medical use, ie, therapy or prophylaxis of a disease or condition in a human or animal subject wherein the disease or condition is mediated by the antigen (or a pathogen comprising the antigen). Binding of the multimer with said OD₄₅₀ indicates that the multimer is useful for assaying for detecting the presence of the antigen or antibodies against the antigen in a bodily fluid sample of a human or animal, eg, a serum, saliva or cell sample obtained from a human or animal, wherein the human or animal (i) is suffering from, has suffered from or is suspected of suffering from a disease or condition that is mediated by the antigen, or (ii) is suffering from, has suffered from or is suspected of suffering from an infection by a pathogen that comprises the antigen, such as a virus, bacterium or fungus (eg, a yeast). Generally, an Ig (eg, IgG) that binds to its cognate antigen with an affinity (Kd) higher than 1, 10, 100 or 1000 mM are not useful as medicaments. In an example, EM1 of the multimer of the invention is an antigen binding site of an Ig (eg, IgG) Fab fragment that binds to the antigen with an affinity (Kd) higher than 0.1, 1, 10, 100 or 1000 mM (eg, higher than 1 or 10 mM). In an example, the multimer of the invention binds to the antigen with an apparent affinity (avidity) of lower than 0.1 mM, 100 nM, 10 nM, 1 nM, 100 pM, 10 pM, 1 pM or 100 fM. These affinities are amenable to medical use. Affinities are may be determined by any standard method, for example by surface plasmon resonance (SPR) or ELISA, or bilayer interferometry (eg, as per the example below). The method may be carried out at rtp, or optionally at 20 or 25 degrees centrigrade and 1 atm and optionally at a pH from 6.5 to 7.5 (eg, at pH 7). Example Biolayer interferometry binding assay: Binding assays may be performed by biolayer interferometry (BLI) using an Octet Red® instrument (FortéBio). Briefly, His6-tagged antigen (eg, S or RBD protein) at 20 to 100 µg/mL in 1x kinetics buffer (1x PBS, pH 7.4, 0.01% BSA and 830.002% Tween 20) are loaded onto Anti-Penta-HIS™ (HIS1K) biosensors and incubated with the indicated concentrations of multimer. The assay comprises five steps: 1) baseline: 60 s with 1x kinetics buffer; 2) loading: 300 s with his6-tagged proteins; 3) baseline: 60 s with 1x kinetics buffer; 4) association: 120 s with samples (Fab or IgG or multimer); and 5) dissociation: 120 s with 1x kinetics buffer. For estimating the exact Kd, a 1:1 binding model is used. Example ELISA assay: ELISAs are performed in duplicates to determine the binding affinities. Recombinant antigen is diluted to 1 ug/ml in ELISA coating buffer (50 mM carbonate/bicarbonate). One hundred ul of 1 ug/ml antigen is added to each well of an ELISA plate and the plates are incubated overnight at 4°C. The plates are washed three times with PBS containing 0.05% Tween-20 before being blocked with 200 ul 1% bovine serum albumin in PBS for 4 hrs at room temperature. The plates are washed three times as before. Products (multimers or other protein to be tested) are serially diluted in PBS containing 0.05% Tween-20. One hundred ul of sample is added to each well and the plates are incubated overnight at 4°C. The plates are washed four times with PBS containing 0.05% Tween-20. One hundred ul of detection antibody (anti-His-HRP, A7058, Sigma; or anti-Human-IgG HRP, 31410, Thermo Fisher Scientific; or Protein L HRP, M00098, Genscript) diluted in blocking buffer (according to the manufacturers’ recommendations) is added to each well and the plates are incubated at room temperature for 2 h. Following four plate washes, 25 ul of TMB substrate solution (Thermo Fisher Scientific) is added to each well. The reaction is terminated after ~15 min by the addition of 25 ul 3 M HCl. The absorbance at 450 nm is read using a CLARIOstar™ microplate reader (BMG Labtech). Example SPR binding assay: The SPR is carried out at a detergent level of no greater than 0.05% by volume, eg, in the presence of P20 (polysorbate 20; eg, Tween-20™) at 0.05% and EDTA at 3 mM. In one example, the SPR is carried out at 25° C. or 37° C. in a buffer at pH7.6, 150 mM NaCl, 0.05% detergent (eg, P20) and 3 mM EDTA. The buffer can contain 10 mM Hepes. In one example, the SPR is carried out at 25° C. or 37° C. in HBS-EP. HBS-EP is available from Teknova Inc (California; catalogue number H8022). In an example, the affinity (eg, of a VH/VL binding site) is determined using SPR by using any standard SPR apparatus, such as by Biacore™ or using the ProteOn XPR36™ (Bio-Rad®). The binding data can be fitted to 1:1 model inherent using standard techniques, eg, using a model inherent to the ProteOn XPR36™ analysis software. Optionally, the multimer binds to the antigen with an OD₄₅₀ from 1 to 3 (such as from 1 to 2 or from 2 to 3) in an ELISA assay in which the antigen is at a concentration of 1 nM in the assay. Optionally, binding of the multimer to the antigen is saturated as determined by OD₄₅₀ in an ELISA assay in which the antigen is at a concentration between 10 and 100 nM in the assay. ELISA herein may be a sandwich ELISA. Optionally, EM1 and/or EM2 comprises an antibody VH/VL pair or an antibody single variable domain (such as a nanobody, VHH or a dAb). Optionally, EM1 and/or EM2 is (a) The spike protein binding site of an antibody selected from CR3022, CR3014, or any other anti-coronavirus antibody; (b) An ACE2 protein which is capable of binding to SARS-CoV-2 virus spike protein; or (c) A TMPRSS2 protein which is capable of binding to SARS-CoV-2 virus spike protein. Optionally, EM1 and/or EM2, comprises an ACE2 extracellular protein. Optionally, the ACE2 protein is human ACE2 protein. For example, an extracellular protein of ACE2 having UNIPROT number Q9BYF1, the sequence of such ACE2 and the extracellular domain thereof being incorporated herein by reference, along with the nucleotide sequence encoding such. In an example, ACE2 extracellular protein comprises or consists of positions 18 to 615 or 18 to 740 of ACE2 having UNIPROT number Q9BYF1, the sequence comprising or consisting of positions 18 to 740 being incorporated herein by reference, along with the nucleotide sequence encoding such. In an example, ACE2 extracellular protein comprises an amino acid sequence that is at least 70, 80, 85, 90, 95 or 99% identical to the amino acid sequence of human ACE2. In an example, ACE2 extracellular protein comprises positions 18 to 615 of human ACE2, wherein the extracellular protein has an amino acid sequence that is at least 70, 80, 85, 90, 95 or 99% identical to the amino acid sequence of human ACE2. In an example, the amino acid sequence of human ACE2 is positions 18 to 740 of ACE2 having UNIPROT number Q9BYF1. Optionally, EM1 and/or EM2, comprises an TMPRSS2 extracellular protein. Optionally, the TMPRSS2 protein is human TMPRSS2 protein. For example, an extracellular protein of TMPRSS2 having UNIPROT number O15393, the sequence of such TMPRSS2 and the extracellular domain thereof being incorporated herein by reference, along with the nucleotide sequence encoding such. In an example, TMPRSS2 extracellular protein comprises or consists of positions 106 to 492 of TMPRSS2 having UNIPROT number O15393, the sequence comprising or consisting of positions 106 to 492 being incorporated herein by reference, along with the nucleotide sequence encoding such. Optionally, EM1 and/or EM2is an antibody VH/VL pair, eg, an scFv. Examples of Epitope Binding Domains Each effector moiety, such as EM1 and EM2 can be an epitope binding moiety, such as an Ig or non-Ig epitope binding domain. In an example, the or each epitope binding moiety or domain is selected from the group consisting of an antibody variable domain (eg, a VL or a VH, an antibody single variable domain (domain antibody or dAb), a camelid VHH antibody single variable domain, a shark immunoglobulin single variable domain (NAV), a Nanobody™ or a camelised VH single variable domain); a T-cell receptor binding domain; an immunoglobulin superfamily domain; an agnathan variable lymphocyte receptor (J Immunol; 2010 Aug l;185(3):1367-74; "Alternative adaptive immunity in jawless vertebrates; Herrin BR & Cooper M D.); a fibronectin domain (eg, an Adnectin™); an antibody constant domain (eg, a CH3 domain, eg, a CH2 and/or CH3 of an Fcab™) wherein the constant domain is not a functional CHI domain (defined as a CHI domain that can associate with a light chain); an scFv; an (scFv)₂; an sc-diabody; an scFab; a centyrin and an epitope binding domain derived from a scaffold selected from CTLA-4 (Evibody™); a lipocalin domain; Protein A such as Z-domain of Protein A (eg, an Affibody™ or SpA); an A-domain (eg, an Avimer™ or Maxibody™); a heat shock protein (such as and epitope binding domain derived from GroEI and GroES); a transferrin domain (eg, a trans-body); ankyrin repeat protein (eg, a DARPin™); peptide aptamer; C-type lectin domain (eg, Tetranectin™); human γ- crystallin or human ubiquitin (an affilin); a PDZ domain; scorpion toxin; and a kunitz type domain of a human protease inhibitor. Further sources of epitope binding moieties are variable domains and VH/VL pairs of antibodies disclosed in WO2007024715 at page 40, line 23 to page 43, line 23. This specific disclosure is incorporated herein by reference as though explicitly written herein to provide basis for epitope binding moieties for use in the present invention and for possible inclusion in claims herein. A "domain" is a folded protein structure which has tertiary structure independent of the rest of the protein. Generally, domains are responsible for discrete functional properties of proteins and in many cases may be added, removed or transferred to other proteins without loss of function of the remainder of the protein and/or of the domain. A "single antibody variable domain" is a folded polypeptide domain comprising sequences characteristic of antibody variable domains. It therefore includes complete antibody variable domains and modified variable domains, for example, in which one or more loops have been replaced by sequences which are not characteristic of antibody variable domains, or antibody variable domains which have been truncated or comprise N- or C-terminal extensions, as well as folded fragments of variable domains which retain at least the binding activity and specificity of the full-length domain. The phrase "immunoglobulin single variable domain" refers to an antibody variable domain (VH, VHH, VL) that specifically binds an antigen or epitope independently of a different V region or domain. An immunoglobulin single variable domain can be present in a format (e.g., homo- or hetero-multimer) with other, different variable regions or variable domains where the other regions or domains are not required for antigen binding by the single immunoglobulin variable domain (i.e., where the immunoglobulin single variable domain binds antigen independently of the additional variable domains). A "domain antibody" or "dAb" is the same as an "immunoglobulin single variable domain" which is capable of binding to an antigen as the term is used herein. An immunoglobulin single variable domain may be a human antibody variable domain, but also includes single antibody variable domains from other species such as rodent (for example, as disclosed in WO 00/29004), nurse shark and Camelid VHH immunoglobulin single variable domains. Camelid VHH sre immunoglobulin single variable domain polypeptides that are derived from species including camel, llama, alpaca, dromedary, and guanaco, which produce heavy chain antibodies naturally devoid of light chains. Such VHH domains may be humanised according to standard techniques available in the art, and such domains are still considered to be "domain antibodies" according to the invention. As used herein "VH includes camelid VHH domains. NA V are another type of immunoglobulin single variable domain which were identified in cartilaginous fish including the nurse shark. These domains are also known as Novel Antigen Receptor variable region (commonly abbreviated to V(NAR) or NARV). For further details see Mol. Immunol.44, 656-665 (2006) and US20050043519A. CTLA-4 (Cytotoxic T Lymphocyte-associated Antigen 4) is a CD28-family receptor expressed on mainly CD4+ T-cells. Its extracellular domain has a variable domain- like Ig fold. Loops corresponding to CDRs of antibodies can be substituted with heterologous sequence to confer different binding properties. CTLA-4 molecules engineered to have different binding specificities are also known as Evibodies. For further details see Journal of Immunological Methods 248 (1-2), 31-45 (2001). Lipocalins are a family of extracellular proteins which transport small hydrophobic molecules such as steroids, bilins, retinoids and lipids. They have a rigid β-sheet secondary structure with a numer of loops at the open end of the conical structure which can be engineered to bind to different target antigens. Anticalins are between 160-180 amino acids in size, and are derived from lipocalins. For further details see Biochim Biophys Acta 1482: 337-350 (2000), US7250297B1 and US20070224633 An affibody is a scaffold derived from Protein A of Staphylococcus aureus which can be engineered to bind to antigen. The domain consists of a three-helical bundle of approximately 58 amino acids. Libraries have been generated by randomisation of surface residues. For further details see Protein Eng. Des. Sel.17, 455-462 (2004) and EP1641818A1. Avimers™ are multidomain proteins derived from the A-domain scaffold family. The native domains of approximately 35 amino acids adopt a defined disulphide bonded structure. Diversity is generated by shuffling of the natural variation exhibited by the family of A- domains. For further details see Nature Biotechnology 23(12), 1556 - 1561 (2005) and Expert Opinion on Investigational Drugs 16(6), 909-917 (June 2007). A transferrin is a monomeric serum transport glycoprotein. Transferrins can be engineered to bind different target antigens by insertion of peptide sequences in a permissive surface loop. Examples of engineered transferrin scaffolds include the Trans-body. For further details see J. Biol. Chem 274, 24066-24073 (1999). Designed Ankyrin Repeat Proteins (DARPins™) are derived from ankyrin which is a family of proteins that mediate attachment of integral membrane proteins to the cytoskeleton. A single ankyrin repeat is a 33 residue motif consisting of two a-helices and a β-turn. They can be engineered to bind different target antigens by randomising residues in the first a-helix and a β-turn of each repeat. Their binding interface can be increased by increasing the number of modules (a method of affinity maturation). For further details see J. Mol. Biol.332, 489-503 (2003), PNAS 100(4), 1700-1705 (2003) and J. Mol. Biol.369, 1015-1028 (2007) and US20040132028A1. Fibronectin is a scaffold which can be engineered to bind to antigen. Adnectins™ consist of a backbone of the natural amino acid sequence of the 10th domain of the 15 repeating units of human fibronectin type III (FN3). Three loops at one end of the β-sandwich can be engineered to enable an Adnectin to specifically recognize a therapeutic target of interest. For further details see Protein Eng. Des. Sel.18, 435- 444 (2005), US20080139791 , WO2005056764 and US6818418B1. Peptide aptamers are combinatorial recognition molecules that consist of a constant scaffold protein, typically thioredoxin (TrxA) which contains a constrained variable peptide loop inserted at the active site. For further details see Expert Opin. Biol. Ther.5, 783-797 (2005). Microbodies are derived from naturally occurring microproteins of 25-50 amino acids in length which contain 3-4 cysteine bridges - examples of microproteins include KalataBI and conotoxin and knottins. The microproteins have a loop which can be engineered to include upto 25 amino acids without affecting the overall fold of the microprotein. For further details of engineered knottin domains, see WO2008098796. Other epitope binding moieties and domains include proteins which have been used as a scaffold to engineer different target antigen binding properties include human γ-crystallin and human ubiquitin (affilins), kunitz type domains of human protease inhibitors, PDZ- domains of the Ras-binding protein AF-6, scorpion toxins (charybdotoxin), C-type lectin domain (tetranectins) are reviewed in Chapter 7 - Non-Antibody Scaffolds from Handbook of Therapeutic Antibodies (2007, edited by Stefan Dubel) and Protein Science 15:14-27 (2006). Epitope binding moieties and domains of the present invention could be derived from any of these alternative protein domains. In one embodiment of the invention a or each EM, epitope binding moiety, domain or antigen-binding site binds to antigen or second epitope with a KD of 1 mM, for example a KD of 10 nM, 1 nM, 500 pM, 200 pM, 100 pM or 10 pM or less (ie, better affinity) to each antigen as measured by Biacore™ or Proteon™, such as the Biacore™ method as described in method 4 or 5 of WO2010136485 or as described elsewhere herein. In one embodiment of the invention a or each variable domain, variable domain pair or antigen-binding site at the N-terminus of a polypeptide, chain or antibody of the invention binds to antigen or first epitope with a KD of 1 mM, for example a KD of 10 nM, 1 nM, 500 pM, 200 pM, 100 pM or 10 pM or less (ie, better affinity) to each antigen as measured by Biacore™ or Proteon™, such as the Biacore™ method as described in method 4 or 5 of WO2010136485 or as described elsewhere herein. In an embodiment, the germline of the vertebrate comprises a single type of said locus. The locus is preferably an antibody locus. The locus may be a heavy chain locus. The locus may be a light chain (eg, kappa or lambda) chain locus. The locus may be a T-Cell Receptor (TCR) locus. The locus may be a TCR alpha chain locus. The locus may be a TCR beta chain locus. The locus may be a TCR gamma chain locus. The locus may be a TCR delta chain locus. In an embodiment, the vertebrate (eg, the spleen or bone marrow thereof) comprises lymphoid progenitor cells that comprise a single type of said locus. In an embodiment, the vertebrate (eg, the spleen or bone marrow thereof) comprises pro-B cells that comprise a single type of said locus. This is useful, since such progenitor or pro-B cells are capable of maturing into B- cells that express a repertoire of polypeptides of the invention (and thus produce multimers of the invention) that differ by their EM1 moieties (eg, when the EM1s are antibody variable domains produced following rearrangement of the first nucleotide sequence in B-cells). By using an Ig locus comprised by the vertebrate, such as when the first sequence is an unrearranged variable region that is capable of recombination to produce a nucleotide sequence encoding EM1, it is possible to harness the sequence maturation mechanisms in vivo, such as affinity maturation and somatic hypermutation. The capability to switch the locus between isotypes also enables harnessing of accompanying maturation naturally found in maturing B- or T-cell populations. For example, the EM1-encoding sequence can be matured by the action of activation-induced cytidine deaminase (AID) and/or terminal deoxynucleotidyl transferase (TdT). This is useful, as is known, when producing repertoires of binding moieties in vivo. For example, therefore, EM1 comprises endogenous (eg, mouse when the vertebrate is a mouse, or rat when the vertebrate is a rat) pattern activation-induced cytidine deaminase (AID) and/or terminal deoxynucleotidyl transferase (TdT) mutation. Optionally, the first nucleotide sequence is an Ig locus variable region. Optionally, the variable region is an unrearranged Ig variable region that is capable of rearrangement to produce a nucleotide sequence encoding EM1. The Ig variable region may be an antibody heavy or light chain variable region. The Ig variable region may be TCR alpha, beta, gamma or delta variable region. The unrearranged variable region is capable of rearrangement to produce a nucleotide sequence encoding EM1, wherein EM1 is a variable domain. This is useful for producing a repertoire of polypeptides in the vertebrate that differ by their EM1 sequences and wherein the polypeptides comprise a SAM. As is known to the skilled addressee, rearrangement of Ig loci is associated with affinity maturation of variable domains encoded by the loci. Thus, rearrangement of the locus of the invention may produce an affinity matured nucleotide sequence encoding EM1. In an embodiment, EM1 comprises endogenous (ie, non-human vertebrate species, preferably mouse) AID-pattern somatic hypermutation. For example, the vertebrate is a mouse and EM1 comprises mouse AID- pattern somatic hypermutation. For example, the germline of the vertebrate comprises the locus. The vertebrate, for example, comprises lymphocytic cells that comprise the locus. For example, the vertebrate comprises B-cells that comprise the locus. For example, the vertebrate comprises T-cells that comprise the locus. Each said cell will express only one type of polypeptide comprising EM1 and SAM, and thus multimers produced by the cell will only comprise one type of polypeptide, copies of which have been multimerised by SAMs. The cells will secrete the multimers into the serum of the vertebrate, whereby the serum will comprise a plurality of different multimers (such as when the first nucleotide sequence of each locus is an unrearranged variable region that is rearranged to encode EM1 (preferably a V domain) in the lymphocytic cells). For example, different EM1s will be present in the repertoire and thus the multimers may differ by their EM1s. This is useful to provide a diversity of EM1s for testing, eg, for binding to a predetermined antigen. In this way, using conventional screening techniques, it is possible to select one or more desired multimers that bind an antigen of choice. For example, the vertebrate is immunised with the antigen and a repertoire of multimers obtained from the vertebrate are screened for antigen binding. As is conventional, it may be desirable for example to screen B-cells to obtain a B-cell (or nucleic acid or a nucleic acid sequence) from the cell that encodes a EM1. This sequence can subsequently be used to construct an expression vector by operably linking the EM1-encoding sequence to a SAM-encoding sequence (preferably the SAM that was present in the locus of the B-cell). The vector can be introduced into a host cell (eg, a CHO, Cos or HEK cell) for expression of polypeptides comprising EM1 and SAM. The polypeptides will self-associate via the SAMs to produce multimers comprising a plurality of copies of EM1. The multimers may be isolated from the cell and optionally formulated to produce a pharmaceutical composition for administration to a human or or animal to treat or prevent a disease or condition mediated by the antigen. It is important to note that in each such lymphocytic cell only one species of polypeptide comprising a SAM is produced. This enables high purity production of a single type of multimer (eg, a tetramer of the polypeptide when the SAM is a tetramerization domain (TD) such as a p53 or TTR (transthyretin) TD). This opens up the possibility, using the present invention, of producing multi-specific products and preferably at high purity. In this case, each polypeptide comprises EM1 and one or more further antigen binding domains. One or more of the domains may capable of specifically binding to the same antigen as EM1, or may bind to a different antigen (thereby the polypeptide and multimers are multi- (eg, bi, tri or tetra-) specific for antigen binding. For example, each polypeptide may comprise EM1 (that is capable of specifically binding to a first antigen) and EM2 (that is capable of specifically binding to a second antigen that is different from the first antigen). This is a significant advancement over the art of producing multi-specific products where chain pairing issues pose serious problems, including low yields of the desired product. Optionally, the locus comprises, in 5’ to 3’ direction, the first nucleotide sequence and the second nucleotide sequence; or the locus comprises, in 5’ to 3’ direction, the second nucleotide sequence and the first nucleotide sequence. Optionally, the locus comprises a third nucleotide sequence for producing a second effector moiety (EM2), wherein the third sequence is 5’ of the first sequence; the third sequence is 3’ of the first sequence; or the third sequence is between the first and second sequences. Optionally, the third nucleotide sequence is an Ig locus variable region. Optionally, the variable region is a rearranged Ig variable region. Optionally, the variable region is an unrearranged Ig variable region that is capable of rearrangement to produce a nucleotide sequence encoding EM2. The Ig variable region may be an antibody heavy or light chain variable region. The Ig variable region may be TCR alpha, beta, gamma or delta variable region. The unrearranged variable region is capable of rearrangement to produce a nucleotide sequence encoding EM2, wherein EM2 is a variable domain. This is useful for producing a repertoire of polypeptides in the vertebrate that differ by their EM2 sequences and wherein the polypeptides comprise a SAM. As is known to the skilled addressee, rearrangement of Ig loci is associated with affinity maturation of variable domains encoded by the loci. Thus, rearrangement of the locus of the invention may produce an affinity matured nucleotide sequence encoding EM2. In an embodiment, EM2 comprises endogenous (ie, non-human vertebrate species, preferably mouse) AID-pattern somatic hypermutation. For example, the vertebrate is a mouse and EM2 comprises mouse AID-pattern somatic hypermutation. Optionally, EM1 is capable of binding to a first antigen or epitope; EM2 is capable of binding to a second antigen or epitope; and a) The first and second antigens are identical; b) The first and second epitopes are identical; c) The first and second antigens are different from each other; d) The first and second epitopes are different from each other. Optionally, the first nucleotide sequence is an unrearranged Ig variable region and the third nucleotide sequence is a rearranged Ig variable region. Thus, the locus comprises a nucleotide sequence of a predetermined moieity for producing an EM2 (eg, antibody single variable domain that is capable of binding to a cognate antigen) and the first nucleotide sequence is capable of rearrangement whereby the vertebrate is capable of producing a repertoire of polypeptides, comprising a repertoire of different first effector moieties, whereby the vertebrate is capable of producing a repertoire of said multimers, wherein said repertoire comprises multimers that differ from each other by their first moieties. In an example, the locus comprises an intronic enhancer (eg, an Eμ or Eiκ) and the third nucleotide sequence is 3’ of (ie, downstream of) the enhancer in the locus. This positioning is useful to minimise mutation of the third sequence, thereby maximising chances of preserving the EM2 sequence in the polypeptide product. Optionally, the locus is an antibody locus. Optionally, the locus is aT-cell receptor locus. The polypeptide may comprise more than 2 EMs, for example it may comprise EM1-EM6 as described further below. Optionally, each said effector moiety comprises a) a protein domain; or b) a peptide; optionally wherein each said effector moiety comprises an Ig variable domain (eg, an antibody VH or VL; or a TCR Vα, Vβ, Vγ or Vδ) or an epitope binding domain. Optionally, each of EM1-EM6 is a protein domain, eg, an antibody variable domain. Optionally, each of EM1-EM6 is a peptide. Optionally, each said effector moiety is a protein domain or a peptide; optionally wherein each said effector moiety is an Ig variable domain or an epitope binding domain. In an embodiment, each polypeptide comprises an antibody variable domain (eg, a VH or VL, such as a V-kappa or a V-lambda) and an antibody constant domain. In an embodiment, each polypeptide comprises a T-cell receptor (TCR) variable domain (eg, a V-alpha or V-beta) and an antibody constant domain. In an embodiment, each polypeptide comprises a T-cell receptor (TCR) variable domain (eg, a V-alpha or V-beta) and TCR constant domain. In an embodiment, the first nucleotide sequence comprises an antibody variable region (eg, a heavy chain locus variable region) comprising (in 5' to 3' direction) one or more V gene segments; optionally one or more D gene segments; and one or more J gene segments, wherein the variable region is capable of rearranging to produce a rearranged VDJ or VJ. In an embodiment, the first nucleotide sequence comprises a T-cell receptor (TCR) variable region comprising (in 5' to 3' direction) one or more TCR V gene segments; optionally one or more D gene segments; and one or more J gene segments, wherein the variable region is capable of rearranging to produce a rearranged VDJ or VJ. Optionally, the first constant region is an endogenous constant region of the vertebrate (eg, an endogenous mu constant region). Optionally, the first constant region comprises one or more antibody C gene segments that are endogenous segments of the vertebrate, optionally wherein the C gene segment(s) are endogenous heavy chain constant region gene segment(s), or a endogenous light chain constant region gene segment(s). Optionally, the second constant region comprises one or more antibody C gene segments that are endogenous segments of the vertebrate, optionally wherein the second constant region is a modified endogenous heavy chain constant region, or a modified endogenous light chain constant region, wherein the modification comprises the insertion of at least a nucleotide sequence encoding a SAM. Optionally, EM1 is a TCR variable domain and the locus comprises (i) the functional TCRBV, D and J gene segments of a human TCR locus from TCRBV19 to TCRBJl-1 inclusive, and optionally up to TCRBJl-6; or (ii) the functional TCRAV and J gene segments of a human TCR locus from TCRAV24 to TCRAJ61 inclusive, and optionally up to TCRAJ1. Optionally, a) the one or more V gene segments are TCRAV segments and the one or more J gene segments are TCRAJ gene segments; b) the one or more V gene segments are TCRBV segments and the one or more J gene segments are TCRBJ gene segments; c) the one or more V gene segments are TCRCV segments and the one or more J gene segments are TCRCJ gene segments; or d) the one or more V gene segments are TCRDV segments and the one or more J gene segments are TCRDJ gene segments. In an alternative, EM1 comprises a rearranged TCR VJ or VDJ. Optionally, each said effector moiety is a single-variable-domain TCR (svd TCR). Optionally, each said effector moiety is an antibody single variable domain, eg, a VH, VL such as a Vκ or a Vλ, VHH or a TCR V domain such as a Vα, Vβ, Vγ or Vδ. In an embodiment, each polypeptide comprises an antibody variable domain (eg, a VH or VL, such as a V-kappa or a V-lambda) and at least one antibody constant domain. Suitable constant domains are CH1, CH2 and CH3. For example, the polypeptide comprises a CH1 (but not a CH2 or CH3); a CH2; a CH3; or an antibody Fc region. For example, the polypeptide comprises in N- to C-terminal direction a constant region comprising a CH1, an optional hinge, a CH2 and a CH3. The hinge may be any hinge disclosed in US2022162285, the disclosure of such hinges and their sequences being incorporated herein for use in the present invention. Optionally, the hinge is devoid of a core region (see, US2022162285 for discussion of this). For example, the polypeptide comprises in N- to C-terminal direction a constant region comprising a CH1 but not a CH2 or CH3. For example, the polypeptide comprises in N- to C-terminal direction a constant region comprising a CL, eg, a Cκ or a Cλ. In an example, the polypeptide comprises:- a) EM1, the constant region and SAM; b) EM1, the constant region, SAM and EM2; c) EM1, the constant region, EM2 and SAM; d) EM1, EM2, the constant region and SAM; e) EM2, EM1, the constant region and SAM; f) EM1, SAM and the constant region; g) SAM, EM1 and the constant region; h) EM1, EM2, SAM and the constant region; or i) EM2, EM1, SAM and the constant region. In any disclosure herein, the or each constant region or domain, the CH1, the CH2, the CH3, the CH2 and CH3 or the Fc is respectively a human constant region or domain, CH1, CH2, CH3, CH2 and CH3 or Fc. For example, the isotype of the constant region or domain, the CH1, the CH2, the CH3, the CH2 and CH3 or the Fc is selected from the group IGHA1*01, IGHA1*02, IGHA1*03, IGHA2*01, IGHA2*02, IGHA2*03, IGHD*01, IGHD*02, IGHE*01, IGHE*02, IGHE*03, IGHE*04, IGHEP1*01, IGHEP1*02, IGHEP1*03, IGHEP1*04, IGHG1*01, IGHG1*02, IGHG1*03, IGHG1*04, IGHG1*05, IGHG1*06, IGHG1*07, IGHG1*08, IGHG1*09, IGHG1*10, IGHG1*11, IGHG1*12, IGHG1*13, IGHG1*14, IGHG2*01, IGHG2*02, IGHG2*03, IGHG2*04, IGHG2*05, IGHG2*06, IGHG2*07, IGHG2*08, IGHG2*09, IGHG2*10, IGHG2*11, IGHG2*12, IGHG2*13, IGHG2*14, IGHG2*15, IGHG2*16, IGHG2*17, IGHG3*01, IGHG3*02, IGHG3*03, IGHG3*04, IGHG3*05, IGHG3*06, IGHG3*07, IGHG3*08, IGHG3*09, IGHG3*10, IGHG3*11, IGHG3*12, IGHG3*13, IGHG3*14, IGHG3*15, IGHG3*16, IGHG3*17, IGHG3*18, IGHG3*19, IGHG3*20, IGHG3*21, IGHG3*22, IGHG3*23, IGHG3*24, IGHG3*25, IGHG3*26, IGHG3*27, IGHG3*28, IGHG3*29, IGHG4*01, IGHG4*02, IGHG4*03, IGHG4*04, IGHG4*05, IGHG4*06, IGHG4*07, IGHG4*08, IGHGP*01, IGHGP*02, IGHGP*03, IGHM*01, IGHM*02, IGHM*03 and IGHM*04 (eg, the constant region is a *01 allele listed in said group, preferably the constant region is a human IGHG1*01 or IGHM*01 constant region). In an alternative, the or each constant region or domain, the CH1, the CH2, the CH3, the CH2 and CH3 or the Fc is a non-human (eg, mammal, rodent, mouse, rat, dog, cat or horse) constant region or domain, CH1, CH2, CH3, CH2 and CH3 or Fc. The polypeptide, in one embodiment, comprises (in N- to C-terminal direction) a first antigen binding site (EM1, eg, a dAb), an antibody CH1 (eg, human IgG1 CH1), a hinge sequence comprising a lower hinge and devoid of a core hinge region (and optionally devoid of an upper hinge region), an antibody Fc region and a SAM (eg, a TD, such as a p53 TD). For example, the core hinge region sequence is a CXXC amino acid sequence. The polypeptide may comprise another antigen binding site (eg a dAb or scFv) between the first binding site and the CH1, between the Fc and SAM and/or C-terminal to the SAM. Optionally, the multimer comprises a plurality (eg, 4 copies) of such polypeptide, for example wherein each polypeptide is paired with a further polypeptide comprising (in N- to C-terminal direction) a second antigen binding site (eg, a dAb), an antibody CL (eg, a human Cκ) and optionally a third antigen binding site. Optionally the binding sites have the same antigen specificity (eg, all bind TNF alpha). In another option, the first and second (and optionally said another binding site) bind to different antigens. The or each binding site can bind any antigen disclosed herein, eg, each binding site binds TNF alpha. In another example, the first antigen binding site is a VH of an antigen binding site of a predetermined antibody that specifically binds to the antigen (and the CH1 is optionally the CH1 of the antibody), and the second binding site of the further polypeptide is a VL of the antigen binding site of the predetermined antibody (and the CL is optionally the CL of the antibody), wherein the VH and VL pair to form a VH/VL binding site which has binding specificity for the antigen. The predetermined antibody may be a marketed antibody. For example, the VH/VL binding site specifically binds to CTLA-4, eg, wherein the predetermined antibody is ipilimumab (or Yervoy™). For example, the VH/VL binding site specifically binds to TNF alpha, eg, wherein the predetermined antibody is adalimumab, golimumab, infliximab (or Humira™, Simponi™ or Remicade™). For example, the VH/VL binding site specifically binds to PD-L1, eg, wherein the predetermined antibody is avelumab (or Bavencio™) or atezolizumab (or Tecentriq™). For example, the VH/VL binding site specifically binds to PD-1, eg, wherein the predetermined antibody is nivolumab (or Opdivo™) or pembrolizumab (or Keytruda™). For example, the VH/VL binding site specifically binds to VEGF, eg, wherein the predetermined antibody is bevacizumab (or Avastin™) or ranibizumab (or Lucentis™). In another example, the polypeptide comprises (in N- to C-terminal direction) a first VEGF binding site, an optional second VEGF binding site, an antibody CH1 (eg, human IgG1 CH1), a hinge sequence comprising a lower hinge and devoid of a core hinge region (and optionally devoid of an upper hinge region), an antibody Fc region and a SAM (eg, a TD, such as a p53 TD). In an example, the first binding site is a Ig domain 2 from VEGFR1 and the second binding site is Ig domain 3 from VEGFR2. In another example, the first binding site is a Ig domain 3 from VEGFR2 and the second binding site is Ig domain 2 from VEGFR2. In an example, the first and second binding domains are (in N- to C-terminal direction) the first and second VEGF binding sites of aflibercept (or Eylea™). In an example, the polypeptide is a heavy chain of an IgG antibody, such any predetermined antibody disclosed herein, except that the antibody is modified by comprising a SAM in each heavy chain; eg, wherein a SAM is at the C-terminus of each heavy chain of the IgG antibody. This advantageously enables multimers of the antibody to form in B-cells of the invention. The B-cells may therefore secrete multimers of an IgG antibody, wherein the antibody comprises heavy chains wherein each heavy chain comprises a C-terminal SAM and the SAMs associate to multimerise copies of the antibody. Suitable predetermined antibodies are ReoPro™; Abciximab; Rituxanh™; Rituximab; Zenapaxh™; Daclizumab; Simulecth™; Basiliximab; Synagis™; Palivizumab; Remicadeh™; Infliximab; Herceptinh™; Trastuzumab; Mylotargh™; Gemtuzumab; Campathh™; Alemtuzumab; Zevalinh™; Ibritumomab; Humirah™; Adalimumab; Xolair™; Omalizumab; Bexxarh™; Tositumomab; Raptivah™; Efalizumab; Erbituxh™; Cetuximab; Avastinh™; Bevacizumab; Tysabrih™; Natalizumab; Actemrah™; Tocilizumab; Vectibixh™; Panitumumab; Lucentish™; Ranibizumab; Solirish™; Eculizumab; Cimziah™; Certolizumab; Simponih™; Golimumab, Ilaris™; Canakinumab; Stelara™; Ustekinumab; Arzerrah™; Ofatumumab; Prolie™; Denosumab; Numaxh™; Motavizumab; ABThraxh™; Raxibacumab; Benlystah™; Belimumab; Yervoyh™; Ipilimumab; Adcetrish™; Brentuximab; Vedotin™; Perjeta™; Pertuzumab; Kadcyla™; Ado-trastuzumab; Gazyva™ and Obinutuzumab. Also disclosed are the generic versions of these and the corresponding INN names – each of which is a suitable predetermined antibody for use as a source of antigen binding sites for use in the present invention. Suitable sequences of VH and VL domains of predetermined antibodies are disclosed in Table 4 of US2022162285, the disclosure of which is incorporated herein by reference. Thus, for example, the multimer of the invention comprises a plurality (eg, 4, 8, 12, 16 or 20) copies of the VH/VL antigen binding site of any of these antibodies, eg, wherein the VH of the binding site is comprised by a polypeptide of the invention that comprises a SAM (eg, a TD) and each polypeptide is paired with a further polypeptide comprising the VL that pairs with the VH, thus forming an antigen binding site. In an example, the polypeptide comprising the SAM also comprises a CH1 which pairs with a CL of the further polypeptide. Optionally, the binding site of the polypeptide of the multimer comprises a VH of the binding site of the antibody and also the CH1 of the antibody (ie, in N- to C-terminal direction the VH-CH1 and SAM). In an embodiment, the polypeptide may be paired with a further polypeptide comprising (in N- to C-terminal direction a VL-CL, eg, wherein the CL is the CL of the antibody). In one embodiment, the predetermined antibody is Avastin. In one embodiment, the predetermined antibody is Actemra. In one embodiment, the predetermined antibody is Erbitux. In one embodiment, the predetermined antibody is Lucentis. In one embodiment, the predetermined antibody is sarilumab. In one embodiment, the predetermined antibody is dupilumab. In one embodiment, the predetermined antibody is alirocumab. In one embodiment, the predetermined antibody is evolocumab. In one embodiment, the predetermined antibody is pembrolizumab. In one embodiment, the predetermined antibody is nivolumab. In one embodiment, the predetermined antibody is ipilimumab. In one embodiment, the predetermined antibody is remicade. In one embodiment, the predetermined antibody is golimumab. In one embodiment, the predetermined antibody is ofatumumab. In one embodiment, the predetermined antibody is Benlysta. In one embodiment, the predetermined antibody is Campath. In one embodiment, the predetermined antibody is rituximab. In one embodiment, the predetermined antibody is Herceptin. In one embodiment, the predetermined antibody is durvalumab. In one embodiment, the predetermined antibody is daratumumab. In another embodiment, the polypeptide comprises (in N- to C-terminal direction) a first antigen binding site (EM1), an optional linker (eg, a G₄S_n, wherein n=1, 2, 3, 4, 5, 6, 7, ot 8, preferably 3), a second antigen binding site (EM2), a hinge sequence comprising a lower hinge and devoid of a core hinge region (and optionally devoid of an upper hinge region), an antibody Fc (eg, an IgG1 Fc) and a SAM (eg, a TD, such as a p53 TD or TTR TD). For example, the core hinge region sequence is a CXXC amino acid sequence. The polypeptide may comprise another antigen binding site (eg a dAb or scFv) between the Fc and SAM and/or C-terminal to the SAM. Optionally, the multimer comprises a plurality (eg, 4 copies) of such polypeptide. Optionally the binding sites have the same antigen specificity (eg, all bind TNF alpha). In another option, the first and second (and optionally said another binding site) bind to different antigens. The or each binding site can bind any antigen disclosed herein, eg, each binding site binds PD-L1, or the first binding site binds PD-L1 and the second binding site binds 41-BB, or the first binding site binds 4-1BB and the second binding site binds PD-L1. The polypeptide, in one embodiment, comprises (in N- to C-terminal direction) a first antigen binding site (eg, a dAb), an optional linker (eg, a G₄S_n, wherein n=1, 2, 3, 4, 5, 6, 7, ot 8, preferably 3), a second antigen binding site (eg, a dAb), an antibody CH1 (eg, human IgG1 CH1) and a SAM (eg, a TD, such as a p53 TD). The polypeptide may comprise another antigen binding site (eg a dAb or scFv) C-terminal to the SAM. Optionally, the multimer comprises a plurality (eg, 4 copies) of such polypeptide, for example wherein each polypeptide is paired with a further polypeptide comprising (in N- to C-terminal direction) a third antigen binding site (eg, a dAb), an optionaly fourth antigen binding site (eg, a dAb), an antibody CL (eg, a human Cκ or Cλ) and optionally a furhter antigen binding site. For example, the fourth and further binding sites are omitted. In another example, the third and fourth binding sites, but not the further binding site, are present. In another example, the third and further (but not the fourth) binding sites are present. Optionally the binding sites have the same antigen specificity (eg, all bind TNF alpha). In another option, the first and second (and optionally said another said binding site) bind to different antigens. The or each binding site can bind any antigen disclosed herein, eg, each binding site binds TNF alpha. In an example, the first and third, or the second and third binding sites pair to form a VH/VL pair that is identical to the VH/VL binding site of an anti-TNF alpha antibody, such as adalimumab, golimumab, infliximab (or Humira™, Simponi™ or Remicade™). In an example, the first and third, or the second and third binding sites pair to form a VH/VL pair that is identical to the VH/VL binding site of an anti-PD-L1 antibody, such as avelumab (or Bavencio™) or atezolizumab (or Tecentriq™). In an example, the first and third, or the second and third binding sites pair to form a VH/VL pair that is identical to the VH/VL binding site of an anti- PD-1 antibody, such as nivolumab (or Opdivo™) or pembrolizumab (or Keytruda™). In an example, the first and third, or the second and third binding sites pair to form a VH/VL pair that is identical to the VH/VL binding site of an anti-VEGF antibody, such as bevacizumab (or Avastin™) or ranibizumab (or Lucentis™). Predetermined antibodies as discussed above can be used as the source of the VH/VL pairs. In an embodiment, each polypeptide comprises a T-cell receptor (TCR) variable domain (eg, a V-alpha or V-beta) and an antibody constant domain. In an embodiment, each polypeptide comprises a T-cell receptor (TCR) variable domain (eg, a V-alpha or V-beta) and TCR constant domain. Optionally, SAM is a tetramerization domain (TD), optionally a p53, TTR, TPR, BCR, L27 TD. In an example, SAM is a p53 TD. In another example, SAM is a TTR TD. The L27 domain, initially identified in the Caenorhabditis elegans Lin-2 and Lin-7 proteins, is a protein interaction module that exists in a large family of scaffold proteins. The domain can function as an organization centre of large protein assemblies required for establishment and maintenance of cell polarity. In an example, SAM is a TD of any one of proteins 1 to 119 listed in Table 1. Human p53 (UniProtKB - P04637 (P53_HUMAN)) acts as a tumor suppressor in many tumor types, inducing growth arrest or apoptosis depending on the physiological circumstances and cell type. It is involved in cell cycle regulation as a trans-activator that acts to negatively regulate cell division by controlling a set of genes required for this process. Human p53 is found in increased amounts in a wide variety of transformed cells. It is frequently mutated or inactivated in about 60% of cancers. Human p53 defects are found in Barrett metaplasia a condition in which the normally stratified squamous epithelium of the lower esophagus is replaced by a metaplastic columnar epithelium. The condition develops as a complication in approximately 10% of patients with chronic gastroesophageal reflux disease and predisposes to the development of esophageal adenocarcinoma. Nine isoforms of p53 naturally occur and are expressed in a wide range of normal tissues but in a tissue-dependent manner. Isoform 2 is expressed in most normal tissues but is not detected in brain, lung, prostate, muscle, fetal brain, spinal cord and fetal liver. Isoform 3 is expressed in most normal tissues but is not detected in lung, spleen, testis, fetal brain, spinal cord and fetal liver. Isoform 7 is expressed in most normal tissues but is not detected in prostate, uterus, skeletal muscle and breast. Isoform 8 is detected only in colon, bone marrow, testis, fetal brain and intestine. Isoform 9 is expressed in most normal tissues but is not detected in brain, heart, lung, fetal liver, salivary gland, breast or intestine. Optionally, the second nucleotide sequence is comprised by an Ig constant region of the locus (optionally wherein the locus is an antibody locus and the second nucleotide sequence is comprised by a gamma constant region). The constant region is preferably a human constant region. Optionally, the locus is an antibody light chain locus and comprises, in 5’ to 3’ direction, a light chain constant region and the second nucleotide sequence encoding SAM, wherein the locus is capable of expressing a light chain polypeptide that comprises EM1, a light chain constant domain and a said SAM. Preferably, the constant region of the locus comprises an exon encoding a Cκ wherein the in the polypeptide the SAM is fused to the C-terminus of the Cκ. Preferably, the constant region of the locus comprises an exon encoding a Cλ wherein the in the polypeptide the SAM is fused to the C-terminus of the Cλ. EM1 may be N-terminal to the constant domain in the polyeptide. Optionally, the locus is an antibody heavy chain locus and comprises, in 5’ to 3’ direction, a heavy chain constant region and the second nucleotide sequence encoding SAM, wherein the locus is capable of expressing a heavy chain polypeptide that comprises EM1, one or more heavy chain constant domains and a said SAM. For example, the constant region encodes a CH1 domain and/or an antibody Fc. Preferably, the constant region of the locus comprises an exon encoding a CH3 wherein the in the polypeptide the SAM is fused to the C-terminus of the CH3. Alternatively, optionally the constant region of the locus comprises an exon encoding a CH1 wherein the in the polypeptide the SAM is fused to the C-terminus of the CH1 (and in this option the polypeptide may be devoid of a CH2 and CH3). Alternatively, optionally the constant region of the locus comprises an exon encoding a CH2 wherein the in the polypeptide the SAM is fused to the C-terminus of the CH2. Alternatively, optionally the constant region of the locus comprises an exon encoding a hinge wherein the in the polypeptide the SAM is fused to the C-terminus of the hinge. Optionally, the locus is an antibody heavy chain locus and comprises, in 5’ to 3’ direction, a) a first constant region of a first isotype (optionally a mu isotype); and b) a second constant region of a second isotype (optionally a non-mu isotype, such as a gamma isotype), wherein the second constant region comprises said second nucleotide sequence; wherein the locus is capable of isotype switching from the first isotype to the second isotype wherein the locus is capable of expressing polypeptides of the second isotype wherein each polypeptide comprises EM1 and a said SAM. In an example, the second constant region comprises at its 3’ end the second nucleotide sequence encoding SAM, optionally wherein the second constant region comprises, in 5’ to 3’ order, an exon encoding a CH3 and the second nucleotide sequence encoding SAM. For example, the second constant region is an antibody delta constant region, wherein the locus is capable of producing a delta chain comprising a SAM, wherein the chain is capable of self-associating to produce producing multivalent IgD multimers. For example, the second constant region is an antibody alpha constant region, wherein the locus is capable of producing an alpha chain comprising a SAM, wherein the chain is capable of self-associating to produce producing multivalent IgA multimers. For example, the second constant region is an antibody delta constant region, wherein the locus is capable of producing a delta chain comprising a SAM, wherein the chain is capable of self-associating to produce producing multivalent IgD multimers. For example, the second constant region is an antibody epsilon constant region, wherein the locus is capable of producing an epsilon chain comprising a SAM, wherein the chain is capable of self-associating to produce producing multivalent IgE multimers. For example, the second constant region is an antibody gamma constant region, wherein the locus is capable of producing a gamma chain comprising a SAM, wherein the chain is capable of self-associating to produce producing multivalent IgG multimers. For example, the second constant region is an antibody gamma-1 constant region, wherein the locus is capable of producing a gamma-1 chain comprising a SAM, wherein the chain is capable of self-associating to produce producing multivalent IgG1 multimers. For example, the second constant region is an antibody gamma-2 constant region, wherein the locus is capable of producing a gamma-2 chain comprising a SAM, wherein the chain is capable of self-associating to produce producing multivalent IgG2 multimers. For example, the second constant region is an antibody gamma-3 constant region, wherein the locus is capable of producing a gamma-3 chain comprising a SAM, wherein the chain is capable of self- associating to produce producing multivalent IgG3 multimers. For example, the second constant region is an antibody gamma-4 constant region, wherein the locus is capable of producing a gamma-4 chain comprising a SAM, wherein the chain is capable of self- associating to produce producing multivalent IgG4 multimers. The vertebrate may be a mouse and the second constant region is a mouse (eg, endogenous mouse) antibody gamma- 2a constant region, wherein the locus is capable of producing a gamma-2a chain comprising a SAM, wherein the chain is capable of self-associating to produce producing multivalent IgG2a multimers. The vertebrate may be a mouse and the second constant region is a mouse (eg, endogenous mouse) antibody gamma-2b constant region, wherein the locus is capable of producing a gamma-2b chain comprising a SAM, wherein the chain is capable of self- associating to produce producing multivalent IgG2b multimers. The vertebrate may be a mouse and the second constant region is a mouse (eg, endogenous mouse) antibody gamma- 2c constant region, wherein the locus is capable of producing a gamma-2c chain comprising a SAM, wherein the chain is capable of self-associating to produce producing multivalent IgG2c multimers. The constant region may be a human constant region. The constant region may comprise exons encoding human C domains. The constant region may be a non-human vertebrate (eg, rodent, eg, mouse) constant region. The constant region may comprise exons encoding non- human vertebrate (eg, rodent, eg, mouse) C domains. The constant region may be an endogenous constant region of the vertebrate. The domains may be endogenous C domains of the vertebrate. Optionally, a cell of the invention is a vertebrate, mammalian, non-human mammalian, rodent, mouse, rat or human cell. Optionally, the cell is a human or rodent (eg, a mouse or a rat) cell. Optionally, the cell or mouse is a 129 or C57BL/6 strain cell or mouse (eg, a hybrid 129 or hybrid C57BL/6 strain cell or mouse). Optionally, the cell or mouse is a hybrid 129/C57BL/6 strain cell or mouse, such as a F1H4 strain cell or mouse. A mouse herein or mouse domain or constant region may be a 129 strain mouse or mouse domain or constant region. A mouse herein or mouse domain or constant region may be a C57BL6 strain mouse or mouse domain or constant region. In an example, the locus comprises two or more of a delta, gamma, epsilon and alpha constant region, wherein each said constant region comprises a 3’-most exon encoding a constant domain (eg, a gamma CH3) and a nucleotide sequence encoding a SAM, wherein the latter sequence is 3’ of the 3’-most exon (eg, fused to the 3’ end of the exon whereby the constant domain can be expressed as C-SAM (in N- to C-terminal direction)). For example, the third sequence is 5’ of the second sequence in the second constant region. In an embodiment, the second constant region comprises a third nucleotide sequence that encodes a second effector moiety (EM2) wherein EM1 and EM2 are capable of binding to different antigens. In one embodiment, the second constant region is a human constant region. In one embodiment, the first constant region is a human constant region and the second constant region is a human constant region. In one embodiment, the vertebrate is a vertebrate of a first species (eg, a rodent, such as a mouse or rat, or a rabbit, preferably a mouse) and the first constant region (eg, a mu constant region) is a constant region of said first species. Optionally, in this embodiment, the second constant region is a human constant region or a constant region of said species. This enables isotype switching to a human constant region (eg, a gamma constant region) comprising a SAM for multimer formation. The first constant region may comprise a first switch sequence that recombines with a second switch sequence that is comprised by the second constant region, whereby isotype switching takes place. Optionally, the first constant region is devoid of a SAM. For example, when the first constant region is a mu constant region, the repertoire of mu isotype antibodies can be produced by the vertebrate and affinity maturation of EM1 (such as a V domain) and isotype switching to the second constant region can usefully thereafter take place. In an example, the second constant region is devoid of a nucleotide sequence encoding a CH1. In this way, the polypeptide encoded by the second constant region cannot chain pair with antibody light chains, whereby multimers are produced that are devoid of antibody light chains. For example, the polypeptide produced by the second constant region may comprise in N- to C-terminal direction EM1 and an antibody heavy chain constant region that is devoid of a CH1 (eg, the constant region comprises an Fc but not a CH1). Optionally, the first constant region comprises a CH1. This enables a repertoire of mu 4-chain antibodies to be produced for selection in the vertebrae before isotype switching to non-mu (preferably gamma) antibody multimers that are devoid of light chains. In an example, the first constant region is a mu isotype constant region and the second constant region is a non-mu constant region, wherein the genome of the vertebrate comprises a light chain locus, wherein (a) the light chain locus comprises (in 5' to 3' direction) a variable region and a constant region for expressing light chains in lymphocytic cells expressing IgM antibodies; and (b) means for turning off light chain expression in lymphocytic cells expressing non-mu antibodies. Optionally, the locus is capable of producing a polypeptide selected from a polypeptide comprising, in N- to C-terminal direction, a) EM1 and SAM; b) SAM and EM1; c) EM1, EM2 and SAM; d) SAM, EM1 and EM2 e) EM1, SAM and EM2; f) EM1, EM2, EM3 and SAM; g) SAM, EM1, EM2 and EM3; h) EM1,EM2 SAM and EM3; i) EM1 SAM, EM2 and EM3; j) EM1, EM2, EM3, EM4 and SAM; k) SAM, EM1, EM2, EM3 and EM4; l) EM1, EM2, EM3 SAM and EM4; m) EM1 SAM, EM2, EM3 and EM4; n) EM1, EM2 SAM, EM3 and EM4; o) EM1, EM2, EM3, EM4, EM5 and SAM; p) SAM, EM1, EM2, EM3, EM4 and EM5; q) EM1, EM2, EM3 SAM, EM4 and EM5; r) EM1, EM2 SAM, EM3, EM4 and EM5; s) EM1, EM2, EM3 SAM, EM4 and EM5; t) EM1, EM2, SAM, EM3, EM4 and EM5; and u) EM1, EM2, EM3, SAM, EM4, EM5 and EM6; wherein each EM is an effector moiety (optionally an Ig variable domain or a peptide). In an embodiment, the polypeptide comprises a peptide linker joining the SAM to the EM that is immediately 5' of the SAM and/or the polypeptide comprises a peptide linker joining the SAM to the EM that is immediately 3' of the SAM. In an embodiment, EM1 is capable of binding to a first antigen or epitope; EM2 is capable of binding to a second antigen or epitope; and a) The first and second antigens are identical; b) The first and second epitopes are identical; c) The first and second antigens are different from each other; d) The first and second epitopes are different from each other. The invention provides:- A non-human vertebrate or a non-human vertebrate cell that comprises a rearranged Ig variable region (eg, an VH or VL region) encoding a V domain, wherein the variable region is ectopically positioned in the genome of the vertebrate or cell, wherein the vertebrate or cell expresses polypeptides each comprising a SAM and a said V domain, wherein the V domains of the polypeptides comprise most commonly a CDR3 length of 11 to 16 (eg, 15) amino acids. By "ectopically" it is meant that the variable region is transcribed from a non-natural genomic location, ie, from a genomic position outside the respective endogenous Ig locus (eg, antibody locus) of the genome of a said non-human vertebrate, eg, from a position outside all of the antibody loci of said genome. Thus, for example the V region is an antibody VH region and is not present in an endogenous IgH locus. Thus, for example the V region is an antibody Vκ region and is not present in an endogenous Igκ locus. Thus, for example the V region is an antibody Vλ region and is not present in an endogenous Igλ locus. Optionally, VDJ or VJ gene segments comprised by the first nucleotide sequence are rearranged ectopically to produce said rearranged variable region in said vertebrate. In an embodiment, the variable region is a heavy chain variable region that is transcribed from a genomic position that is not at an endogenous heavy chain locus (preferably, the endogenous heavy chain locus is inactivated for expression of endogenous VH domains). In an embodiment, the variable region is an antibody variable region that is transcribed from a genomic position that is not at an endogenous antibody locus. In an embodiment, the genomic position of the locus of the invention is not in any antibody locus and/or the position is at an antibody locus (eg, an IgH, IgK or IgA locus). The described CDR3 length approximates the range seen in humans, thus is useful for human medicament application. There is provided:- A non-human vertebrate (eg, a mouse) (eg, a vertebrate as described herein) or a non-human vertebrate cell (eg, a mouse cell) whose genome comprises a locus for expression of antibody heavy chains, the locus comprising a) an unrearranged human variable region comprising human variable region gene segments for expression of a repertoire of human variable domains (EM1 moieties); b) an endogenous mu constant region for expression of IgM antibody heavy chains comprising endogenous mu heavy chain constant domains and human variable domains; and c) (i) a humanised non-mu constant region (eg, a gamma region, such as a gamma-1 region) downstream of the mu constant region for expression of non-mu antibody heavy chains comprising human non-mu constant domains and human variable domains, wherein the non-mu constant region comprises a nucleotide sequence encoding a SAM; or (ii) a non-human vertebrate (eg, mouse) non-mu constant region (eg, a gamma region, such as a gamma-1 region) downstream of the mu constant region for expression of non-mu antibody heavy chains comprising non- human vertebrate (eg, mouse) non-mu constant domains and human variable domains, wherein the non-mu constant region comprises a nucleotide sequence encoding a SAM; wherein the unrearranged variable region is provided as a targeted insertion of the human variable region gene segments upstream of the endogenous mu constant region in an endogenous IgH locus such that the variable region gene segments are able to recombine for expression and selection in the context of an endogenous mu constant region. There is provided:- A non-human vertebrate (eg, a mouse or a rat) (eg, a vertebrate as described herein) or a non- human vertebrate cell whose genome comprises an antibody heavy chain locus comprising (in 5′ to 3′ direction) a variable region, a first switch, an endogenous mu constant region, a second switch and a human non-mu (eg, gamma, such as gamma-1) constant region, wherein the non-mu constant region comprises a nucleotide sequence encoding a SAM; wherein the heavy chain locus of each cell is capable of undergoing switching from IgM to the non-mu (eg, IgG) isotype for the production of non-mu heavy chains comprising a rearranged V domain and a SAM. Optionally, each said cell comprises an isotype switched locus obtainable (or obtained) by switching the locus to the non-mu isotype, wherein the switched locus comprises a rearranged variable region encoding EM1 and produced by rearrangement of a variable region comprised by the first nucleotide sequence, wherein each said cell produces a respective polypeptide comprising an EM1 and a SAM. Preferably, EM1 of the polypeptide is an affinity matured protein domain, eg, an Ig variable domain. An affinity matured domain can be obtained, for example, using isotype switching of the locus as described herein. A cell or a plurality of cells, wherein each cell is a) an isolated lymphocytic cell (optionally a B- or T-cell) obtainable from a vertebrate of any preceding claim, each comprises a locus as defined in any preceding claim for producing polypeptides comprising EM1 and a SAM.; or b) an embryonic stem cell or pluripotent stem cell that can develop into a vertebrate according to any preceding claim. Optionally, the plurality comprises at least 10², 10³, 10⁴, 10⁵, 10⁶, 10⁷, 10⁸, 10⁹, 10¹⁰, 10¹¹, 10¹², 10¹³ or 10¹⁴ of said cells. Optionally, said cell is a mouse or rat cell; or said cells are mouse cells or rat cells. Optionally, a cell herein is a mouse or rat ES cell that is capable of developing into a mouse or rat respectively, wherein the mouse or rat is capable of expressing a plurality of heavy chains. The chains may comprise a plurality of antigen specificities or affinities, each said heavy chain being capable of pairing with an antibody chain (eg, a kappa or lambda light chain) encoded by the first locus to produce paired chains that comprise an antigen binding site. Optionally, a cell herein is an antibody-producing cell for the production of multimers that specifically bind to an antigen. The cell is, eg, a CHO, HEK, MEF, COS or HeLa cell. Optionally, each said cell is capable of secreting a multimer of its respective polypeptide, wherein copies of the polypeptide are associated by SAMs. Preferably, each said cell secretes a multimer. Typically, multimers will comprise glycosylation. For example, the vertebrate is a mouse and the multimers will comprise mouse- pattern glycosylation. There is provided:- A lymphocyte cell (optionally obtainable from a vertebrate described herein) and comprising a locus for expressing a polypeptide, wherein the locus comprises (in 5' to 3' direction) a) an unrearranged Ig variable region comprising at least one V gene segment, optionally at least one D gene segment, and at least one J gene segment, wherein the variable region is capable of rearrangement for expressing a rearranged Ig variable domain; b) an Ig locus intronic enhancer; and c) a first Ig constant region encoding a first C domain; wherein the locus comprises a nucleotide sequence encoding a first self-associating mulimerization domain (SAM) (such as a self-associating tetramerization domain (TD)), the locus being operable to express a polypeptide comprising said rearranged V domain, SAM and C domain. A method of producing a nucleic acid for expressing a polypeptide comprising an EM1 and SAM, the method comprising (a) determining a nucleotide sequence of an EM1-encoding sequence comprised by a locus of a vertebrate described herein, wherein EM1 is a V domain and the first nucleotide sequence comprises a rearranged V region encoding said V domain; and (b) producing a nucleic acid by operably connecting the sequence of step (a) with a nucleotide sequence encoding a SAM. In an embodiment, the method is a method of producing an expression vector comprising the nucleic acid, wherein the vector is capable of expressing a polypeptide comprising EM1 and a SAM. Preferably, the vector is capable of expressing the polypeptide in a eukaryotic cell, eg, a mammalian, human or rodent cell, such as a HEK293, Cos or CHO cell. Optionally, the nucleic acid encodes a polypeptide comprising EM1, SAM and one or more antibody constant domains (eg, an antibody Fc region). Optionally, the constant domain(s) are human constant domains. For example, the domains are one or more of a CH1, CH2 and CH3, for example, the domains comprise CH2 and CH3. For example, the constant domain comprises a CL (eg, a C-kappa or C-lambda domain). Optionally, the polypeptide comprises, in N- to C-terminal direction, (a) EM1, the constant domain(s) and SAM (optionally, EM1, Fc and SAM); or (b) EM1, SAM and the constant domain(s) (optionally, EM1, SAM and Fc). Optionally, the polypeptide is an antibody heavy chain that comprises, in N- to C-terminal direction, EM1, Fc and SAM, wherein EM1 is optionally a VH. There is provided:- An expression vector or a host cell comprising a nucleic acid obtained by the method of producing a nucleic acid. Preferably, the vector is a DNA vector. Optionally, the cell further comprises a nucleic acid encoding an antibody light chain, wherein the light chain comprises a second rearranged variable domain (eg, a VL), wherein the cell is capable of producing a multimer comprising two copies of a 4- chain antibody, wherein each antibody comprises a first copy of the heavy chain associated with a first copy of the light chain, a second copy of the heavy chain associated with a second copy of the light chain, and wherein the SAMs of the heavy chains associate together thereby forming the multimer. Optionally, in each antibody of the variable domains of the first heavy and light chains form a first antigen binding site; and the variable domains of the second heavy and light chains form a second antigen binding site. In an embodiment, the first and second antigens are identical. There is provided:- A method of producing an antibody multimer, the method comprising expressing the multimer in a host cell described herein, and optionally isolating the multimer from the cell. Optionally, the method further comprises formulating the isolated multimer to produce a pharmaceutical composition comprising the multimer and a pharmaceutically acceptable diluent, excipient or carrier. Optionally, the multimer(s) or composition is comprised by a medical container or device. A medical container may be a container with sterile contents. For example, the container or device is an injection device (eg, a syringe), vial or IV bag. A method of producing a polypeptide comprising EM1 and a SAM, or a method of producing a multimer of copies of a polypeptide comprising EM1 and a SAM, wherein EM1 is a protein domain that is capable of binding to a first antigen, the method comprising immunising a vertebrate described herein, wherein the vertebrate expresses polypeptides or multimers comprising EM1 (that is capable of specifically binding to the antigen) and SAM, and obtaining a said polypeptide or multimer. A method of obtaining a nucleotide sequence encoding an EM1, wherein EM1 is a protein domain that is capable of binding to a first antigen, the method comprising immunising a vertebrate described herein with the first antigen, wherein the variable region of the locus undergoes rearrangement to produce a nucleotide sequence encoding EM1 (that is capable of specifically binding to the antigen), and obtaining said nucleotide sequence from a B-cell of the immunised vertebrate wherein the B-cell expresses a polypeptide comprising EM1. Optionally, the method comprises producing an expression vector for expressing a polypeptide comprising EM1 and a SAM, wherein the method comprises operably linking the obtained nucleotide sequence to a nucleotide sequence encoding SAM. There is provided:- A method for producing an expression vector for expressing a polypeptide comprising EM1 and a SAM, wherein the method comprises operably linking a nucleotide sequence that has been obtained by the method of obtaining a nucleotide sequence to a nucleotide sequence encoding SAM. In the vertebrate, cell(s), vector or cell described herein, optionally each said polypeptide comprises an EM1 and a tetramerization domain (TD), wherein a first, a second, a third and a fourth copy of the polypeptide are capable of associating together by their TDs to form a tetramer comprising at least 4 copies of EM1. Optionally, each TD is a p53 TD or a homologue or orthologue thereof. Optionally, each TD is a TTR TD or a homologue or orthologue thereof. There is provided:- A multimer comprising a first, a second, a third and a fourth copy of the polypeptide, wherein EM1 is an Ig variable domain comprising an amino acid sequence that is encoded by the first nucleotide sequence of a vertebrate described herein wherein the first nucleotide sequence comprises an Ig V region that has been rearranged in the vertebrate, wherein the vertebrate is a mouse and the amino acid sequence is encoded by a nucleotide sequence that comprises mouse pattern activation-induced cytidine deaminase (AID) and/or terminal deoxynucleotidyl transferase (TdT) mutation, optionally wherein the polypeptide comprises one or more human constant domains (such as an Fc region). For example, the polypeptide comprises in N- to C-terminal direction EM1, a Fc region and a SAM There is provided:- A polyclonal polypeptide multimer population, wherein each multimer is a multimer of at least 3 or 4 (preferably 4) copies of a respective polypeptide comprising an Ig V domain and a SAM, wherein SAMs comprised by each multimer are associated together, and wherein the population comprises at least 10 different types of said V domain of the polypeptides. Optionally, the population of multimers comprises at least 100, 1000, 10000 or 100000 different types of said V domain. For example, the V domains are recombinants of the same V, D and J gene segments but differ in their amino acid sequences by mutation caused by affinity maturation in a non-human vertebrate (eg, a mouse) and each V domain is obtainable from a vertebrate of the invention that has been immunised with an antigen, wherein at least some of the V domains are capable of binding to the antigen. Optionally, each V domain is encoded by a respective nucleotide sequence that comprises rodent (eg, mouse or rat) pattern activation-induced cytidine deaminase (AID) and/or terminal deoxynucleotidyl transferase (TdT) mutation, and optionally which is obtainable from a vertebrate described herein that has been immunised by an antigen, wherein a plurality of said V domains are capable of binding to the antigen. Such binding may be specific binding to the antigen. There is provided:- A method for identifying or obtaining an antibody variable domain, a nucleotide sequence encoding an antibody variable domain, or an expression vector or host cell that is capable of expressing the domain, wherein the domain is capable of specifically binding to a target antigen, the method comprising (a) contacting the population described herein with said antigen; (b) binding multimers comprised by said population to said antigen; and (c) isolating or identifying one or more multimers that bind to the antigen, or isolating or identifying a said V domain of such a multimer; or identifying a nucleotide sequence encoding a V domain of such a multimer; and (d) optionally connecting the sequence to a nucleotide sequence encoding a SAM to produce a nucleic acid for expressing a polypeptide comprising the V domain and SAM; and optionally expressing and isolating said polypeptide or multimers thereof. There is provided:- A non-human vertebrate (eg, mouse) blastocyst or pre-morula embryo implanted with an ES or iPS cell comprising a locus described herein, wherein the blastocyst or pre-morula embryo implanted with an ES or iPS cell is capable of developing into a vertebrate described herein. Optionally, the homology arms are homologous (or identical) to first and second sequences comprised by a constant region (eg, a gamma constant region) of said endogenous Ig locus, whereby the SAM can be inserted into the endogenous constant region. For example, the vector nucleic acid comprises a 5' homology arm and a 3' homology arm flanking said sequence encoding a SAM. In an embodiment, the Ig locus is an antibody IgH locus and the 5' arm is homologous (or identical) to a sequence of a CH3 sequence of a constant region (eg, a gamma or gamma-1) region of the endogenous antibody locus. Additionally, the 3' arm may be homologous (or identical) to a sequence of the constant region that is 3' of the CH3. In this way, the SAM may be inserted 3' (such as immediately 3') of the CH3 in the endogenous constant region. The SAM may thus be expressed as a fusion with CH3 in the resultant polypeptide. The sequence of the 5' arm may be isogenic with the sequence of the Ig locus. The sequence of the 3' arm may be isogenic with the sequence of the Ig locus. In an example, the sequence of one or both arms is a mouse 129 DNA sequence (and optionally the Ig locus is a mouse 129 strain Ig locus). There is provided:- A method of obtaining a humanised and affinity matured antigen-specific antibody heavy chain or a multimer thereof, the method comprising producing the heavy chain in vivo in a non-human vertebrate (eg, a mouse or a rat) comprising a functional activation induced cytidine deaminase (AID) by immunising the vertebrate with the antigen and obtaining somatic hypermutation and isotype switching in a B-cell of the vertebrate from an endogenous mu isotype to a human non-mu isotype, wherein an affinity matured antigen- specific antibody heavy chain is produced and expressed by the vertebrate, the non-mu constant domains of the heavy chain being human constant domains encoded by a non-mu constant region of the vertebrate and the heavy chain comprising a SAM encoded by the non- mu constant region, wherein copies of the heavy chain self-associated via SAMs to produce multimers and the method further comprises isolating at least one said multimer. Optionally, the SAM is fused to the 3’-most amino acid of a CH3 of the heavy chain. In the cell, vertebrate, multimer, population or method the polypeptide may comprise an antibody heavy chain, the chain comprising in N- to C-terminal direction a rearranged V domain, an antibody Fc region and a SAM, wherein 4 copies of the polypeptide and 4 copies of a cognate light chain are associated together into a multimer, wherein the multimer comprises 2 copies of a 4-chain antibody wherein the heavy chains of the antibodies are associated together by 4 SAMs. As is known, in a 4-chain antibody a first copy of the heavy chain is associated with a first copy of the light chain, a second copy of the heavy chain associated with a second copy of the light chain and the HL chain dimers are associated together to form the 4-chain antibody. In the invention, in an embodiment, therefore a SAM is provided at the C-terminus of each H chain and the SAMs of a first and second antibody associate together to produce a multimer comprising 4 binding sites. There is provided:- A nucleic acid (eg, a YAC) comprising a transgene for microinjection into a non-human vertebrate ES cell, the transgene comprising a locus as defined herein. The transgene can integrate (eg, randomly integrate) into the genome of the ES cell. The resultant cell is capable of developing into a vertebrate of the invention. In an embodiment, an ES cell herein is a rodent, eg, a mouse or rat, preferably a mouse, cell. There is provided:- A nucleic acid vector comprising a nucleic acid, wherein the nucleic acid comprises homology arms for recombination with the genome of a non-human vertebrate ES cell, wherein a nucleotide sequence encoding a SAM is between the homology arms, whereby said recombination is capable of inserting the nucleotide sequence into an endogenous Ig locus of the cell 3’ of an intronic enhancer thereof. The sequences of the homology arms are sufficiently identical to sequence of the endogenous Ig locus downstream of the intronic enhancer, whereby homologous recombination between those sequences and the arms causes insertion of the SAM-encoding sequence into the locus. The nucleic acid vector may be a human artificial chromosome (HAC), yeast artificial chromosome (YAC), P1-derived artificial chromosome (PAC) or bacterial artificial chromosome (BAC). For example, the vector is a bacterial artificial chromosome (BAC). EM1 can be an effector domain (eg, a protein domain) or a peptide. In an embodiment, the protein multimer is a multimer of at least first, second, third and fourth copies of an effector domain (eg, a protein domain) or a peptide, wherein the multimer is multimerised by first, second, third and fourth self-associating tetramerization domains (TDs) which are associated together, wherein each tetramerization domain is comprised by a respective engineered polypeptide comprising one or more copies of said protein domain or peptide. In an example, each TD is a TD of any one of proteins 1 to 119 listed in Table 1. In an example, each TD is a transthyreitin (TTR) TD or a homologue or orthologue thereof. For example, the TTR is humanTTR, eg, TTR having Uniprot number P02766. In an example, each TD is a p53 TD or a homologue or orthologue thereof. In an example, each TD is a NHR2 TD or a homologue or orthologue thereof. In an example, each TD is a p63 TD or a homologue or orthologue thereof. In an example, each TD is a p73 TD or a homologue or orthologue thereof. In an example, each TD is not a NHR2 TD. In an example, each TD is not a p53 TD. In an example, each TD is not a p63 TD. In an example, each TD is not a p73 TD. In an example, each TD is not a p53, 63 or 73 TD. In an example, each TD is not a NHR2, p53, 63 or 73 TD. By being “associated together” or self-associating, the SAMS or TDs multimerise copies (such as first, second, third and fourth copies) of the polypeptide to provide a multimer protein, for example, a multimer that can be expressed intracellulary in a eukaryotic or mammalian cell (eg, a HEK293 cell) and/or which can be extracellularly secreted from a eukaryotic or mammalian cell (eg, a HEK293 cell) and/or which is soluble in an aqueous medium (eg, a eukaryotic or mammalian cell (eg, a HEK293 cell) culture medium). Examples are TTR TD, NHR TD, p53 TD, p63 TD and p73 TD (eg, human NHR TD, p53 TD, p63 TD and p73 TD) or an orthologue or homologue thereof. In an example, the TD is not a p53 TD (or homologue or orthologue thereof), eg, it is not a human p53 TD (or homologue or orthologue thereof). In an example, the TD is a NHR2 TD or a homologue or orthologue thereof, but excluding a p53 TD or a homologue or orthologue thereof. In an example, the TD is a human NHR2 TD or a homologue or orthologue thereof, but excluding a human p53 TD or a homologue or orthologue thereof. In an example, the TD is human NHR2. In an example, the amino acid sequence of the TD is at least 80, 85, 90, 95, 96, 97, 98 or 99% identical to the sequence of human NHR2. In an example, the domain or peptide is not naturally comprised by a polypeptide that also comprise a NHR2 TD. In an example, all of the domains of the polypeptide are human. In an example, the engineered polypeptide comprises a first said domain or peptide and a TD, wherein the first domain or peptide is spaced by at least 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900 or 1000 contiguous amino acids from the SAM or TD, wherein there is no further said domain or peptide between the first domain or peptide and the SAM or TD. In an example, the multimer (eg, tetramer of said polypeptide) comprises 4 (but no more than 4) TDs (eg, identical TDs) and 4, 8, 12 or 16 (but no more than said 4, 8, 12 or 16 respectively) copies of said domain or peptide. In an example, each TD and each said domain or peptide is human. In an example, the multimer comprises first, second, third and fourth identical copies of a polypeptide, the polypeptide comprising a TD and one (but no more than one), two (but no more than two), or more copies of the said protein domain or peptide. There may be provided advantageously a plurality of multimers (eg, a plurality of tetramers or octamers) that comprise only one (and no more than one) type of engineered polypeptide, wherein the multimers are monospecific (but multivalent) for antigen binding, or alternatively bi- or multi-specific for antigen binding. Each polypeptide comprises an EM1 obtained or obtainable from a vertebrate of the invention. Thus, there may be provided a composition comprising plurality of these multimers (eg, a plurality of tetramers or octamers, each polypeptide being at least tetra-valent for antigen binding and (i) bi-specific (ie, capable of specifically binding to 2 different antigens) or (ii) mono-specific and at least tetravalent for antigen binding. Herein, where antigen binding is mentioned this can be pMHC binding when the domain (EM1) is a TCR V domain. Advantageously, the plurality is in pure form (ie, not mixed with multimers (eg, tetramers or octamers) that comprise more than one type of polypeptide monomer. In an example, the multimer comprises at least 2 different types of antigen binding site. In an example, the multimer is bi-specific, tri-specific or tetra-specific. In an example, the multimer has an antigen binding site or pMHC binding site valency of 4, 6, 8, 10 or 12, preferably 4 or 8. In an example, a multimer of the composition is an isolated multimer, trimer or tetramer. In an example, the multimer, trimer or tetramer consists of copies of said engineered polypeptide. Optionally the multimer, trimer or tetramer comprises 4 or 8 but not more than 4 or 8 copies respectively of the engineered polypeptide. In an example, a peptide MHC (pMHC) is a class I or class II pMHC. By “engineered” is meant that the polypeptide is not naturally-occurring, for example the protein domain or peptide is not naturally comprised by a polypeptide that also comprises said SAM or TD. Each said EM1, EM2, protein domain or peptide may be a biologically active domain or peptide (eg, biologically active in humans or animals), such as a domain that specifically binds to an antigen or peptide-MHC (pMHC), or wherein the domain is comprised by an antigen or pMHC binding site. In an alternative, the domain or peptide is a carbohydrate, glucose or sugar-regulating agent, such as an incretin or an insulin peptide. In an alternative, the domain or peptide is an inhibitor or an enzyme or an inhibitor of a biological function or pathway in humans or animals. In an alternative, the domain or peptide is an iron-regulating agent. Thus, in an example, each protein domain or peptide is selected from an antigen or pMHC binding domain or peptide; a hormone; a carbohydrate, glucose or sugar-regulating agent; an iron-regulating agent; and an enzyme inhibitor. Optionally, EM1 comprises an antibody or TCR binding site, such as a scFv or scTCR. The immunoglobulin superfamily (IgSF) is a large protein superfamily of cell surface and soluble proteins that are involved in the recognition, binding, or adhesion processes of cells. Molecules are categorized as members of this superfamily based on shared structural features with immunoglobulins (also known as antibodies); they all possess a domain known as an immunoglobulin domain or fold. Members of the IgSF include cell surface antigen receptors, co-receptors and co-stimulatory molecules of the immune system, molecules involved in antigen presentation to lymphocytes, cell adhesion molecules, certain cytokine receptors and intracellular muscle proteins. They are commonly associated with roles in the immune system. T-cell receptor (TCR) domains can be Vα (eg. paired with a Vβ), Vβ (eg. paired with a Vα), Vγ (eg, paired with a Vδ) or Vδ (eg, paired with a Vγ). Optionally, each polypeptide comprises first and second copies of said protein domain or peptide, wherein the polypeptide comprises in (N- to C-terminal direction) (i) a first of said copies – TD – the second of said copies; (ii) TD – and the first and second copies; or (iii) said first and second copies – TD. Optionally, the TDs are NHR2 TDs and the domain or peptide is not a NHR2 domain or peptide; or wherein the TDs are p53 TDs and the domain or peptide is not a p53 domain or peptide. Optionally, the polypeptide comprises one or more copies of a second type of protein domain or peptide, wherein the second type of protein domain or peptide is different from the first protein domain or peptide. For example, the polypeptide comprises in N-terminal direction (i) P1– TD –P2; or (ii) TD – P1-P2, wherein P1=a copy of a domain or peptide of the first type (ie, the type of domain or peptide of the multimer of Aspect 1); and P2=a copy of a domain or peptide of said second type. Optionally, EM1 and/or EM2 are domains, wherein the domains are immunoglobulin superfamily domains. Optionally, EM1, EM2, the domain or peptide is an antibody variable or constant domain (eg, an antibody single variable domain), a TCR variable or constant domain, an incretin, an insulin peptide, or a hormone peptide. Optionally, the multimer comprises first, second, third and fourth identical copies of a said engineered polypeptide, the polypeptide comprising a TD and one (but no more than one), two (but no more than two) or more copies of the said protein domain or peptide. Optionally, the polypeptide comprises an antibody or TCR variable domain (V1) and a NHR2 TD. Optionally, the polypeptide comprises (in N- to C-terminal direction) (i) V1-an optional linker-NHR2 TD; (ii) V1-an optional linker-NHR2 TD-optional linker-V2; or (iii) V1-an optional linker-V2 – optional linker - NHR2 TD, wherein V1 and V2 are TCR variable domains and are the same or different, or wherein V1 and V2 are antibody variable domains and are the same or different. For example, V1 and V2 are antibody single variable domains. Optionally, each polypeptide comprises (in N- to C-terminal direction) V1-an optional linker- NHR2 TD, wherein V1 is an antibody or TCR variable domain and each engineered polypeptide is paired with a respective second engineered polypeptide that comprises V2, wherein V2 is a an antibody or TCR variable domain respectively that pairs with V1 to form an antigen or pMHC binding site, and optionally one polypeptide comprises an antibody Fc, or comprises antibody CH1 and the other polypeptide comprises an antibody CL that pairs with the CH1. Optionally, the TD comprises (i) an amino acid sequence identical to SEQ ID NO: 10 or 126 or at least 80% identical thereto; or (ii) an amino acid sequence identical to SEQ ID NO: 120 or 123 or at least 80% identical thereto. The sequences in this paragraph are those disclosed in US11,453,726, which sequences are incorporated herein by reference for use with the present invention. Optionally, the multimer comprises a tetramer or octamer of an antigen binding site of an antibody, wherein the binding site comprises EM1, and the antibody is selected from the group consisting of ReoPro™; Abciximab; Rituxan™; Rituximab; Zenapax™; Daclizumab; Simulect™; Basiliximab; Synagis™; Palivizumab; Remicade™; Infliximab; Herceptin™; Mylotarg™; Gemtuzumab; Campath™; Alemtuzumab; Zevalin™; Ibritumomab; Humira™; Adalimumab; Xolair™; Omalizumab; Bexxar™; Tositumomab; Raptiva™; Efalizumab; Erbitux™; Cetuximab; Avastin™; Bevacizumab; Tysabri™; Natalizumab; Actemra™; Tocilizumab; Vectibix™; Panitumumab; Lucentis™; Ranibizumab; Soliris™; Eculizumab; Cimzia™; Certolizumab; Simponi™; Golimumab, Ilaris™; Canakinumab; Stelara™; Ustekinumab; Arzerra™; Ofatumumab; Prolia™; Denosumab; Numax™; Motavizumab; ABThrax™; Raxibacumab; Benlysta™; Belimumab; Yervoy™; Ipilimumab; Adcetris™; Brentuximab; Vedotin™; Perjeta™; Pertuzumab; Kadcyla™; Ado-trastuzumab; Keytruda™, Opdivo™, Gazyva™ and Obinutuzumab. For example, a said EM1, EM2, protein domain of the engineered polypeptide is a V domain (a VH or VL) of an antibody binding site of an antibody selected from said group, wherein the multimer comprises a further V domain (a VL or VH respectively) that pairs with the V domain of the engineered polypeptide to form the antigen binding site of the selected antibody. Advantageously, therefore, the invention provides tetramers or octamers of a binding site of said selected antibody, which beneficially may have improved affinity, avidity and/or efficacy for binding its cognate antigen or for treating or preventing a disease or condition in a human or animal wherein the multimer is administered thereto to bind the cognate antigen in vivo. For example, the multimer, tetramer or octamer comprises 4 copies of an antigen binding site of an antibody, wherein the antibody is adalimumab, sarilumab, dupilumab, bevacizumab (eg, AVASTIN™), cetuximab (eg, ERBITUX™), tocilizumab (eg, ACTEMRA™) or trastuzumab (HERCEPTIN™). In an alternative the antibody is an anti-CD38 antibody, an anti-TNFa antibody, an anti-TNFR antibody, an anti-IL-4Ra antibody, an anti-IL-6R antibody, an anti- IL-6 antibody, an anti-VEGF antibody, an anti-EGFR antibody, an anti-PD-1 antibody, an anti-PD-L1 antibody, an anti-CTLA4 antibody, an anti-PCSK9 antibody, an anti-CD3 antibody, an anti-CD20 antibody, an anti-CD138 antibody, an anti-IL-1 antibody. In an alternative the antibody is selected from the antibodies disclosed in WO2007024715 at page 40, line 23 to page 43, line 23, the disclosure of which is incorporated herein by reference. A binding site herein may, for example, be a ligand (eg, cytokine or growth factor, eg, VEGF or EGFR) binding site of a receptor (eg, KDR or Flt). A binding site herein may, for example, be a binding site of Eyelea™ , Avastin™ or Lucentis™, eg, for ocular or oncological medical use in a human or animal. When the ligand or antigen is VEGF, the mutlimer, tetramer or octamer may be for treatment or prevention of a caner or ocular condition (eg, wet or dry AMD or diabetic retinopathy) or as an inhibitor of neovascularisation in a human or animal subject. EM1 may be an insulin peptide, incretin peptide or peptide hormone. EM2 may be an insulin peptide, incretin peptide or peptide hormone. The multimer may be an isolated tetramer or octamer of a TCR binding site, insulin peptide, incretin peptide or peptide hormone; or a plurality of said tetramers or octamers. Several important peptide hormones are secreted from the pituitary gland. The anterior pituitary secretes three hormones: prolactin, which acts on the mammary gland; adrenocorticotropic hormone (ACTH), which acts on the adrenal cortex to regulate the secretion of glucocorticoids; and growth hormone, which acts on bone, muscle, and the liver. The posterior pituitary gland secretes antidiuretic hormone, also called vasopressin, and oxytocin. Peptide hormones are produced by many different organs and tissues, however, including the heart (atrial-natriuretic peptide (ANP) or atrial natriuretic factor (ANF)) and pancreas (glucagon, insulin and somatostatin), the gastrointestinal tract (cholecystokinin, gastrin), and adipose tissue stores (leptin). In an example, the peptide hormone of the invention is selected from prolactin, ACTH, growth hormone (somatotropin), vasopressin, oxytocin, glucagon, insulin, somatostatin, cholecystokinin, gastrin and leptin (eg, selected from human prolactin, ACTH, growth hormone, vasopressin, oxytocin, glucagon, insulin, somatostatin, cholecystokinin, gastrin and leptin). In an example, the incretin is a GLP-1, GIP or exendin-4 peptide. In embodiments, the multimer is selected from the following engineered tetramers and octamers:- An isolated tetramer or octamer of an incretin. An isolated tetramer or octamer of an insulin peptide. An isolated tetramer or octamer of a GLP-1 (glucagon-like peptide-1 (GLP-1) peptide. An isolated tetramer or octamer of a GIP (glucose-dependent insulinotropic polypeptide) peptide. An isolated tetramer or octamer of an exendin (eg, exendin-4) peptide. An isolated tetramer or octamer of a peptide hormone. An isolated tetramer or octamer of a prolactin or prolactin peptide. An isolated tetramer or octamer of a ACTH or ACTH peptide. An isolated tetramer or octamer of a growth hormone or growth hormone peptide. An isolated tetramer or octamer of a vasopressin or vasopressin peptide. An isolated tetramer or octamer of an oxytocin or oxytocin peptide. An isolated tetramer or octamer of a glucagon or glucagon peptide. An isolated tetramer or octamer of a insulin or insulin peptide. An isolated tetramer or octamer of a somatostatin or somatostatin peptide. An isolated tetramer or octamer of a cholecystokinin or cholecystokinin peptide. An isolated tetramer or octamer of a gastrin or gastrin peptide. An isolated tetramer or octamer of a leptin or leptin peptide. An isolated tetramer or octamer of an antibody binding site (eg, a scFv or Fab). An isolated tetramer or octamer of a TCR binding site (eg, a scTCR). An isolated tetramer or octamer of a TCR Vα/Vβ binding site. An isolated tetramer or octamer of a TCR Vγ/Vδ binding site. An isolated tetramer or octamer of an antibody single variable domain binding site. An isolated tetramer or octamer of an FcAb binding site. In an example of any of these tetramers or octamers, the domain or peptide is human. In an example of any of these tetramers or octamers, the tetramer or octamer comprises a NHR2 TD (eg, a human NHR2). In an example of any of these tetramers or octamers, the tetramer or octamer comprises a p53 TD (eg, a human p53 TD). In an example of any of these tetramers or octamers, the tetramer or octamer comprises a p63 TD (eg, a human p63 TD). In an example of any of these tetramers or octamers, the tetramer or octamer comprises a p73 TD (eg, a human p73 TD). In an example of any of these tetramers or octamers, the tetramer or octamer comprises a tetramer of TDs (eg, human NHR2 TDs), whereby the domains or peptides form a multimer of 4 or 8 domains or peptides. In an example, the plurality is at least 90% pure for the multimer, eg, is not in mixture with multimers of said binding site or peptide wherein the multimers comprise more than one type of polypeptide monomer. In an example the multimer, tetramer or octamer of said composition is secretable from a HEK293T (or other eukaryotic, mammalian, CHO or Cos) cell in stable form as indicated by a single band at the molecular weight expected for said multimer, tetramer or octamer on a PAGE gel using a sample of supernatant from such cells and detected using Western Blot. Optionally, the multimer is a tetramer or octamer of (a) TCR V domains or TCR binding sites, wherein the tetramer or octamer is soluble in aqueous solution (eg, an aqueous eukaryotic cell growth medium or buffer); (b) antibody single variable domains, wherein the tetramer or octamer is soluble in aqueous solution (eg, an aqueous eukaryotic cell growth medium or buffer); (c) TCR V domains or TCR binding sites, wherein the tetramer or octamer is capable of being intracellularly and/or extracellularly expressed by HEK293 cells; or (d) antibody variable domains (eg, antibody single variable domains), wherein the tetramer or octamer is capable of being intracellularly and/or extracellularly expressed by HEK293 cells. An example of the medium is SFMII growth medium supplemented with L-glutamine (eg, complete SFMII growth medium supplemented with 4 mM L-glutamine). In an example, the medium is serum-free HEK293 cell culture medium. In an example, the medium is serum- free CHO cell culture medium. Optionally, the tetramer or octamer is bi-specific for antigen or pMHC binding. Optionally, the domains are identical. Optionally, the multimer, tetramer or octamer of the composition comprises eukaryotic cell glycosylation. For example the glycosylation is CHO cell glycosylation. For example the glycosylation is HEK (eg, HEK293, such as HEK293T) cell glycosylation. For example the glycosylation is Cos cell glycosylation. For example the glycosylation is Picchia cell glycosylation. For example the glycosylation is Sacchaaromyces cell glycosylation. There is provided: A plurality of said multimers, tetramers or octamers. A pharmaceutical composition comprising the multimer(s), tetramer(s) or octamer(s) and a pharmaceutically acceptable carrier, diluent or excipient. A cosmetic, foodstuff, beverage, cleaning product, detergent comprising the multimer(s), tetramer(s) or octamer(s). A said (and optionally isolated) polypeptide or a monomer (optionally isolated) of a multimer, tetramer or octamer described herein. Optionally, the polypeptide comprises (in N- to C-terminal direction) a variable domain (V1) – a constant domain (C) (eg, a CH1 or Fc) – optional linker – TD. Optionally, the polypeptide is a (and optionally isolated) polypeptide (P1) which comprises (in N- to C-terminal direction):- (a) TCR V1 –TCR C1 – antibody C (eg, CH, CH1 (such as IgG CH1) or CL (such as Cλ or a Cκ)) – optional linker – TD, wherein (i) V1 is a Vα and C1 is a Cα; (ii) V1 is a Vβ and C1 is a Cβ; (iii) V1 is a Vγ and C1 is a Cγ; or (iv) V1 is a Vδ and C1 is a Cδ; or (b) TCR V1 – antibody C (eg, CH, CH1 (such as IgG CH1) or CL (such as Cλ or a Cκ)) – optional linker – TD, wherein (i) V1 is a Vα; (ii) V1 is a Vβ; (iii) V1 is a Vγ; or (iv) V1 is a Vδ; or (c) antibody V1 – antibody C (eg, CH, CH1 (such as IgG CH1) or CL (such as Cλ or a Cκ)) – optional linker – TD, wherein (i) V1 is a VH; or (ii) V1 is a VL (eg, a Vλ or a Vκ); or (d) antibody V1 – optional antibody C (eg, CH, CH1 (such as IgG CH1) or CL (such as Cλ or a Cκ)) – antibody Fc (eg, an IgG Fc) – optional linker – TD, wherein (i) V1 is a VH; or (ii) V1 is a VL (eg, a Vλ or a Vκ); or (e) antibody V1 – antibody CL (eg, a Cλ or a Cκ) – optional linker – TD, wherein (i) V1 is a VH; or (ii) V1 is a VL (eg, a Vλ or a Vκ); or (f) TCR V1 –TCR C1 – optional linker – TD, wherein (i) V1 is a Vα and C1 is a Cα; (ii) V1 is a Vβ and C1 is a Cβ; (iii) V1 is a Vγ and C1 is a Cγ; or (iv) V1 is a Vδ and C1 is a Cδ. In (a) or (b), in an example, the TCR V is comprised by an single chain TCR binding site (scTCR) that specifically binds to a pMHC , wherein the binding site comprises TCR V-linker - TCRV. In an example, the engineered polypeptide comprises (in N- to C-terminal direction) (i) V1 – linker – V - optional C - optional linker – TD, or (ii) Va – linker – V1 - optional C - optional linker – TD, wherein Va is a TCR V domain and C is an antibody C domain (eg, a CH1 or CL) or a TCR C. Preferably, the antibody C is CH1 (eg, IgG CH1). In an example the multimer, tetramer or octamer has a size of no more than 155 kDa, eg, wherein said protein domain is an antibody variable domain comprising a CDR3 of at least 16, 17, 18, 19, 20, 21 or 22 amino acids, such as a Camelid CDR3 or bovine CDR3. In an example, the multimer, tetramer or octamer comprises TCR binding sites and antibody binding sites. For example, each polypeptide comprises a TCR V (eg, comprised by a scTCR that specifically binds a pMHC) and an antibody V (eg, comprised by a scFv or paired with a second V domain comprised by a said second polypeptide to form a V/V paired binding site that specifically binds to an antigen). In an example, the pMHC comprises a RAS peptide. In an example the antigen is selected from the group consisting of PD-1, PD-L1 or any other antigen disclosed herein. For example, the antigen is PD-1 and the pMHC comprises a RAS peptide. Optionally, the polypeptide P1 is paired with a further polypeptide (P2), wherein P2 comprises (in N- to C-terminal direction):- (g) TCR V2 –TCR C2 – antibody CL (eg, a Cλ or a Cκ), wherein P1 is according to (a) recited in Aspect 28 and (i) V2 is a Vα and C2 is a Cα when P1 is according to (a)(ii); (ii) V2 is a Vβ and C2 is a Cβ when P1 is according to (a)(i); (iii) V2 is a Vγ and C2 is a Cγ when P1 is according to (a)(iv); or (iv) V2 is a Vδ and C2 is a Cδ when P1 is according to (a)(iii); or (h) TCR V2 – antibody CL (eg, a Cλ or a Cκ), wherein P1 is according to (b) recited in Aspect 28 and (i) V2 is a Vα when P1 is according to (b)(ii); (ii) V2 is a Vβ when P1 is according to (b)(i); (iii) V2 is a Vγ when P1 is according to (b)(iv); or (iv) V2 is a Vδ when P1 is according to (b)(iiii); or (i) Antibody V2 – CL (eg, a Cλ or a Cκ), wherein P1 is according to (c) recited in Aspect 28 and (i) V2 is a VH when P1 is according to (c)(ii); or (ii) V2 is a VL (eg, a Vλ or a Vκ) when P1 is according to (c)(i); or (j) Antibody V2 – optional CL (eg, a Cλ or a Cκ), wherein P1 is according to (d) recited in Aspect 28 and (i) V2 is a VH when P1 is according to (d)(ii); or (ii) V2 is a VL (eg, a Vλ or a Vκ) when P1 is according to (d)(i); or (k) Antibody V2 – CH1 (eg, IgG CH1), wherein P1 is according to (e) recited in Aspect 28 and (i) V2 is a VH when P1 is according to (e)(ii); or (ii) V2 is a VL (eg, a Vλ or a Vκ) when P1 is according to (e)(i); or (l) TCR V2 –TCR C2, wherein P1 is according to (f) recited in Aspect 28 and (i) V2 is a Vα and C2 is a Cα when P1 is according to (f)(ii); (ii) V2 is a Vβ and C2 is a Cβ when P1 is according to (f)(i); (iii) V2 is a Vγ and C2 is a Cγ when P1 is according to (f)(iii); or (iv) V2 is a Vδ and C2 is a Cδ when P1 is according to (f)(iv). Optionally, V1 and V2 form a paired variable domain binding site that is capable of specifically binding to an antigen or pMHC. In an example, V1 and V2 are variable domains of an antibody, eg, selected from the group consisting of ReoPro™; Abciximab; Rituxan™; Rituximab; Zenapax™; Daclizumab; Simulect™; Basiliximab; Synagis™; Palivizumab; Remicade™; Infliximab; Herceptin™; Mylotarg™; Gemtuzumab; Campath™; Alemtuzumab; Zevalin™; Ibritumomab; Humira™; Adalimumab; Xolair™; Omalizumab; Bexxar™; Tositumomab; Raptiva™; Efalizumab; Erbitux™; Cetuximab; Avastin™; Bevacizumab; Tysabri™; Natalizumab; Actemra™; Tocilizumab; Vectibix™; Panitumumab; Lucentis™; Ranibizumab; Soliris™; Eculizumab; Cimzia™; Certolizumab; Simponi™; Golimumab, Ilaris™; Canakinumab; Stelara™; Ustekinumab; Arzerra™; Ofatumumab; Prolia™; Denosumab; Numax™; Motavizumab; ABThrax™; Raxibacumab; Benlysta™; Belimumab; Yervoy™; Ipilimumab; Adcetris™; Brentuximab; Vedotin™; Perjeta™; Pertuzumab; Kadcyla™; Ado-trastuzumab; Keytruda™, Opdivo™, Gazyva™ and Obinutuzumab. In one embodiment, the antibody is Avastin In one embodiment, the antibody is Actemra In one embodiment, the antibody is Erbitux In one embodiment, the antibody is Lucentis In one embodiment, the antibody is sarilumab In one embodiment, the antibody is dupilumab In one embodiment, the antibody is alirocumab In one embodiment, the antibody is evolocumab In one embodiment, the antibody is pembrolizumab In one embodiment, the antibody is nivolumab In one embodiment, the antibody is ipilimumab In one embodiment, the antibody is remicade In one embodiment, the antibody is golimumab In one embodiment, the antibody is ofatumumab In one embodiment, the antibody is Benlysta In one embodiment, the antibody is Campath In one embodiment, the antibody is rituximab In one embodiment, the antibody is Herceptin In one embodiment, the antibody is durvalumab In one embodiment, the antibody is daratumumab In an example, V1 is capable (itself when a single variable domain, or when paired with V2) of specifically binding to an antigen selected from the group consisting of ABCF1; ACVR1; ACVR1B; ACVR2; ACVR2B; ACVRL1; ADORA2A; Aggrecan; AGR2; AICDA; AWI; AIG1; AKAP1; AKAP2; AIYIH; AMHR2; ANGPT1; ANGPT2; ANGPTL3; ANGPTL4; ANPEP; APC; APOC1; AR; AZGP1 (zinc-a-glycoprotein); B7.1; B7.2; BAD; BAFF; BAG1; BAI1; BCL2; BCL6; BDNF; BLNK; BLRl (MDR15); BlyS; BM Pl; BMP2; BMP3B (GDFIO); BMP4; BMP6; BM P8; BMPRIA; BMPRIB; BM PR2; BPAG1 (plectin); BRCA1; CI9orflO (IL27w); C3; C4A; C5; C5R1; CANT1; CASP1; CASP4; CAV1; CCBP2 (D6 / JAB61); CCL1 (1-309); CCL11 (eotaxin); CCL13 (MCP-4); CCL15 (MIP-id); CCL16 (HCC- 4); CCL17 (TARC); CCL18 (PARC); CCL19 (M IP-3b); CCL2 (MCP-1); MCAF; CCL20 (MIP-3a); CCL21 (MIP-2); SLC; exodus-2; CCL22 (MDC / STC-1); CCL23 (M PIF-1); CCL24 (MPIF-2 I eotaxin-2); CCL25 (TECK); CCL26 (eotaxin-3); CCL27 (CTACK /ILC) ; CCL28; CCL3 (MIP-la); CCL4 (M IP-lb); CCL5 (RANTES); CCL7 (MCP-3); CCL8 (mcp- 2); CCNA1; CCNA2; CCND1; CCNE1; CCNE2; CCR1 (CKR1 / HM145); CCR2 (mcp-1RB / RA);CCR3 (CKR3 / CMKBR3); CCR4; CCR5 (CM KBR5 / ChemR13); CCR6 (CMKBR6 / CKR-L3 / STRL22 / DRY6); CCR7 (CKR7 / EBI1); CCR8 (CM KBR8 / TER1 / CKR-L1); CCR9 (GPR-9-6); CCRL1 (VSHK1); CCRL2 (L-CCR); CD164; CD19; CD1C; CD20; CD200; CD-22; CD24; CD28; CD3; CD37; CD38; CD3E; CD3G; CD3Z; CD4; CD40; CD40L; CD44; CD45RB; CD52; CD69; CD72; CD74; CD79A; CD79B; CD8; CD80; CD81; CD83; CD86; CDH1 (E-cadherin); CDH10; CDH12; CDH13; CDH18; CDH19; CDH20; CDH5; CDH7; CDH8; CDH9; CDK2; CDK3; CDK4; CDK5; CDK6; CDK7; CDK9; CDKN1A (p2IWapl/Cipl); CDKN1B (p27Kipl); CDKNIC; CDKN2A (pl6INK4a); CDKN2B; CDKN2C; CDKN3; CEBPB; CER1; CHGA; CHGB; Chitinase; CHST10; CKLFSF2; CKLFSF3; CKLFSF4; CKLFSF5; CKLFSF6; CKLFSF7; CKLFSF8; CLDN3; CLDN7 (claudin-7); CLN3; CLU (clusterin); CMKLR1; CMKOR1 (RDC1); CNR1; COL18A1; COL1A1; COL4A3; COL6A1; CR2; CRP; CSFl (M-CSF); CSF2 (GM-CSF); CSF3 (GCSF); CTLA4; CTNNBl (b-catenin); CTSB (cathepsin B); CX3CL1 (SCYDi) ; CX3CR1 (V28); CXCL1 (GROl); CXCLIO (IP-10); CXCL11 (l-TAC / IP-9); CXCL12 (SDF1); CXCL13; CXCL14; CXCL16; CXCL2 (GR02); CXCL3 (GR03); CXCL5 (ENA-78 I LIX); CXCL6 (GCP-2); CXCL9 (MIG); CXCR3 (GPR9/CKR-L2); CXCR4; CXCR6 (TYMSTR ISTRL33 I Bonzo); CYB5; CYC1; CYSLTR1; DAB2IP; DES; DKFZp451J0118; DNCL1; DPP4; E2F1; ECGF1; EDG1; EFNAI; EFNA3; EFNB2; EGF; EGFR; ELAC2; ENG; EN01; EN02; EN03; EPHB4; EPO; ERBB2 (Her-2); EREG; ERK8; ESR1; ESR2; F3 (TF); FADD; FasL; FASN; FCER1A; FCER2; FCGR3A; FGF; FGF1 (aFGF); FGF10; FGF11; FGF12; FGF12B; FGF13; FGF14; FGF16; FGF17; FGF18; FGF19; FGF2 (bFGF); FGF20; FGF21; FGF22; FGF23; FGF3 (int-2); FGF4 (HST); FGF5; FGF6 (HST-2); FGF7 (KGF); FGF8; FGF9; FGFR3; FIGF (VEGFD); FILl (EPSILON); FILl (ZETA); FU12584; FU25530; FLRTl (fibronectin); FLTl; FOS; FOSLl (FRA-I); FY (DARC); GABRP (GABAa); GAGEB1; GAGEC1; GALNAC4S-65T; GATA3; GDF5; GFI1; GGT1; GM-CSF; GNAS1; GNRHl; GPR2 (CCRIO); GPR31; GPR44; GPR81 (FKSG80); GRCCIO (CIO); GRP; GSN (Gelsolin); GSTPl; HAVCR2; HDAC4; EDAC5; HDAC7A; HDAC9; HGF; HIF1A; HIP1; histamine and histamine receptors; HLA-A; HLA-DRA; HM74; HMOX1; HUMCYT2A; ICEBERG; ICOSL; 1D2; IFN-a; IFNA1; IFNA2; IFNA4; IFNA5; IFNA6; IFNA7; IFNB1; IFNgamma; TFNW1; IGBP1; IGF1; IGF1R; IGF2; IGFBP2; IGFBP3; IGFBP6; IL-1; IL10; IL10RA; IL10RB; IL11; IL11RA; IL-12; IL12A; IL12B; IL12RB1; IL12RB2; 1L13; IL13RA1; IL13RA2; 1L14; 1L15; IL15RA; IL16; 1L17; IL17B; IL17C; IL17R; 1L18; IL18BP; IL18R1; IL18RAP; 1L19; ILIA; IL1B; IL1F10; IL1F5; IL1F6; IL1F7; IL1F8; IL1F9; IL1HY1; IL1R1; IL1R2; IL1RAP; IL1RAPL1; IL1RAPL2; IL1RL1;IL1RL2 IL1RN; 1L2; 1L20; IL20RA; IL21R; 1L22; 1L22R; 1L22RA2; 1L23; 1L24; 1L25; 1L26; 1L27; 1L28A; 1L28B; 1L29; IL2RA; IL2RB; IL2RG; 1L3; 1L30; IL3RA; 1L4; IL4R; 1L5; IL5RA; 1L6; IL6R; IL6ST (glycoprotein 130); 1L7; TL7R; 1L8; IL8RA; IL8RB; IL8RB; 1L9; IL9R; ILK; INHA; INHBA; INSL3; INSL4; IRAKI; IRAK2; ITGA1; ITGA2; 1TGA3; ITGA6 (a6 integrin); ITGAV; ITGB3; ITGB4 (b 4 integrin); JAG1; JAK1; JAK3; JUN; K6HF; KAI1; KDR; MTLG; KLF5 (GC Box BP); KLF6; KLK10; KLK12; KLK13; KLK14; KLK15; KLK3; KLK4; KLK5; KLK6; KLK9; KRT1; KRT19 (Keratin 19); KRT2A; KRTHB6 (hair-specific type II keratin); LAMA5; LEP (leptin); Lingo-p75; Lingo-Troy; LPS; LTA (TNF-b); LTB; LTB4R (GPR16); LTB4R2; LTBR; MACMARCKS; MAG or Omgp; MAP2K7 (c-Jun); MDK; M IB1; midkine; M IF; M IP-2; MK167 (Ki-67); MMP2; M MP9; MS4A1; MSMB; MT3 (metallothionectin-ifi); MTSS 1; M UC 1 (mucin); MYC; MYD88; NCK2; neurocan; NFKB 1; NFKB2; NGFB (NGF); NGFR; NgR-Lingo; NgR-Nogo66 (Nogo); NgR-p75; NgR-Troy; NM E1 (NM23A); NOX5; NPPB; NROB1; NROB2; NR1D1; NR1D2; NR1H2; NR1H3; NR1H4; NR1I2; NR1I3; NR2C1; NR2C2; NR2E1; NR2E3; NR2F1; NR2F2; NR2F6; NR3C1; NR3C2; NR4A1; NR4A2; NR4A3; NR5A1; NR5A2; NR6A1; NRP1; NRP2; NT5E; NTN4; ODZ1; OPRD1; P2RX7; PAP; PARTI; PATE; PAWR; PCA3; PCNA; PDGFA; PDGFB; PECAM1; PF4 (CXCL4); PGF; PGR; phosphacan; PIAS2; PIK3CG; PLAU (uPA); PLG; PLXDC1; PPBP (CXCL7); PPID; PR1; PRKCQ; PRKD1; PRL; PROC; PROK2; PSAP; PSCA; PTAFR; PTEN; PTGS2 (COX-2); PTN; RAC2 (p2IRac2); RARB; RGS1; RGS13; RGS3; RNF110 (ZNF144); ROB02; S100A2; SCGB1D2 (lipophilin B); SCGB2A1 (mammaglobin 2); SCGB2A2 (mammaglobin 1); SCYE1 (endothelial Monocyte-activating cytokine); SDF2; SERPINA1; SERPINIA3; SERPINB5 (maspin); SERPINE1 (PAT-i); SERPINF1; SHBG; SLA2; SLC2A2; SLC33A1; SLC43A1; SLIT2; SPPl; SPRRIB (Spri); ST6GAL1; STABl; STAT6; STEAP; STEAP2; TB4R2; TBX21; TCPIO; TDGF1; TEK; TGFA; TGFB1; TGFB1I1; TGFB2; TGFB3; TGFBI; TGFBR1; TGFBR2; TGFBR3; TH1L; THBS1(thrombospondin-1); THBS2; THBS4; THPO; TIE (Tie-i); T]MP3; tissue factor; TLRIO; TLR2; TLR3; TLR4; TLR5; TLR6; TLR7; TLR8; TLR9; TNF; TNF-a; TNFAIP2 (B94); TNFAIP3; TNFRSF11A; TNFRSF1A; TNFRSF1B; TNFRSF21; TNFRSF5; TNFRSF6 (Fas); TNFRSF7; TNFRSF8; TNFRSF9; TNFSFIO (TRAIL); TNFSF11 (TRANCE); TNFSF12 (AP03L); TNFSF13 (April); TNFSF13B; TNFSF14 (HVEM-L); TNFSF15 (VEGI); TNFSF18; TNFSF4 (0X40 ligand); TNFSF5 (CD40 ligand); TNFSF6 (FasL); TNFSF7 (CD27 ligand); TNFSF8 (CD30 ligand); TNFSF9 (4-lBB ligand); TOLLIP; Toll-like receptors; TOP2A (topoisomerase lia); TP53; TPM 1; TPM2; TRADD; TRAF1; TRAF2; TRAF3; TRAF4; TRAF5; TRAF6; TREM 1; TREM2; TRPC6; TSLP; TWEAK; VEGF; VEGFB; VEGFC; versican; VHL C5; VLA-4; XCL1 (lymphotactin); XCL2 (SCM-lb); XCR1 (GPR5 / CCXCR1); YY1; and ZFPM2. For example in any Configuration, option, embodiment or example of the invention, the multimer, tetramer or octamer specifically binds to first and second epitopes or antigens, each of which is selected from the group consisting of EpCAM and CD3; CD19 and CD3; VEGF and VEGFR2; VEGF and EGFR; CD138 and CD20; CD138 and CD40; CD20 and CD3; CD38 and CD138; CD38 and CD20; CD38 and CD40; CD40 and CD20; CD19 and CD20; CD-8 and IL-6; PDL-1 and CTLA-4; CTLA-4 and BTN02; CSPGs and RGM A; IGF1 and IGF2; IGF1 and/or 2 and Erb2B; IL-12 and IL-18; IL-12 and TWEAK; IL-13 and ADAM8; IL-13 and CL25; IL-13 and IL-lbeta; IL-13 and IL-25; IL-13 and IL-4; IL-13 and IL-5; IL-13 and IL-9; IL-13 and LHR agonist; IL-13 and MDC; IL-13 and MIF; IL-13 and PED2; IL-13 and SPRR2a; IL-13 and SPRR2b; IL-13 and TARC; IL-13 and TGF-beta; IL-1 alpha and IL- 1 beta; MAG and RGM A; NgR and RGM A; NogoA and RGM A; OMGp and RGM A; RGM A and RGM B; Te38 and TNF alpha; TNF alpha and IL-12; TNF alpha and IL-12p40; TNF alpha and IL-13; TNF alpha and IL-15; TNF alpha and IL-17; TNF alpha and IL-18; TNF alpha and IL-1 beta; TNF alpha and IL-23; TNF alpha and M IF; TNF alpha and PEG2; TNF alpha and PGE4; TNF alpha and VEGF; and VEGFR and EGFR; TNF alpha and RANK ligand; TNF alpha and Blys; TNF alpha and GP130; TNF alpha and CD-22; and TNF alpha and CTLA-4. For example, the first epitope or antigen is selected from the group consisting of CD3; CD16; CD32; CD64; and CD89; and the second epitope or antigen is selected from the group consisting of EGFR; VEGF; IGF-1R; Her2; c-Met (aka HGF); HER3; CEA; CD33; CD79a; CD19; PSA; EpCAM; CD66; CD30; HAS; PSMA; GD2; ANG2; IL-4; IL-13; VEGFR2; and VEGFR3. In an example, V1 is capable (itself when a single variable domain, or when paired with V2) of specifically binding to an antigen selected from the group consisting of human IL-1A, IL- 1β, IL-1RN, IL-6, BLys, APRIL, activin A, TNF alpha, a BMP, BMP2, BMP7, BMP9, BMP10, GDF8, GDF11, RANKL, TRAIL, VEGFA, VEGFB or PGF; optionally the multimer comprises a cytokine amino acid sequence (eg, C-terminal to a TD), such as IL-2 or an IL2- peptide; and the multimer, tetramer or octamer is for treating or preventing a cancer in a human subject. In an example the said effector or protein domain is capable of binding to such an antigen; optionally the multimer comprises a cytokine amino acid sequence (eg, C- terminal to a TD), such as IL-2 or an IL2-peptide; and the multimer, tetramer or octamer is for treating or preventing a cancer in a human subject. There is provided:- A nucleic acid encoding a said polypeptide or monomer described herein, optionally wherein the nucleic acid is comprised by an expression vector for expressing the polypeptide. In an example, the nucleic acid is a DNA, optionally operably connected to or comprising a promoter for expression of the polypeptide or monomer. In another example the nucleic acid is a RNA (eg, mRNA). There is provided:- A eukaryotic host cell comprising the nucleic acid or vector for intracellular and/or secreted expression of the multimer, tetramer, octamer, engineered polypeptide or monomer. Use of a nucleic acid or vector in a method of manufacture of protein multimers for producing intracellularly expressed and/or secreted multimers, wherein the method comprises expressing the multimers in and/or secreting the multimers from eukaryotic cells comprising the nucleic acid or vector. Use of a nucleic acid or vector in a method of manufacture of protein multimers for producing glycosylated multimers in eukaryotic cells comprising the nucleic acid or vector. Mammalian glycosylation of the multimers of the composition of the invention is useful for producing medicines comprising or consisting of the multimers, tetramers or octamers of the invention for medical treatment or prevention of a disease or condition in a mammal, eg, a human. The invention thus provides such a method of use as well as the multimer, tetramer or octamer of the invention for this purpose. Similarly, intracellular and/or secreted expression in one or more host cells (or cell lines thereof) that are mammalian according to the invention is useful for producing such medicines. Particularly useful is such expression in HEK293, CHO or Cos cells as these are commonly used for production of medicaments. In an embodiment, the invention comprises a detergent or personal healthcare product comprising a multimer, tetramer or octamer of the invention. In an embodiment, the invention comprises a foodstuff or beverage comprising a multimer, tetramer or octamer of the invention. In an example, the multimer, monomer, dimer, trimer, tetramer, octamer, polypeptide, composition, mixture, use or method of the present invention is for an industrial or domestic use, or is used in a method for such use. For example, it is for or used in agriculture, oil or petroleum industry, food or drink industry, clothing industry, packaging industry, electronics industry, computer industry, environmental industry, chemical industry, aeorspace industry, automotive industry, biotechnology industry, medical industry, healthcare industry, dentistry industry, energy industry, consumer products industry, pharmaceutical industry, mining industry, cleaning industry, forestry industry, fishing industry, leisure industry, recycling industry, cosmetics industry, plastics industry, pulp or paper industry, textile industry, clothing industry, leather or suede or animal hide industry, tobacco industry or steel industry. There is provided:- The multimer, tetramer or octamer for medical use. Optionally, each tetramer has a size of no more than 200, 160, 155 or 150 kDa. Optionally, each multimer is tetra- or octavent for (i) an antibody V; (ii) an antibody Fab; (iii) an antibody dAb; (iv) an antibody scFv; or (v) a TCR V. The invention provides a claim multimer (eg, tetramer) of NHR2 or p53 (or another TD disclosed herein) fused at its N- and/or C-terminus to an amino acid sequence (eg, a peptide, protein domain or protein). For example, latter sequence is selected from a TCR (eg, TCRα, TCRβ, Cα or Cβ), cytokine (eg, interleukin, eg, IL-2, IL-12, IL-12 and IFN), antibody fragments (eg, scFv, dAb or Fab) and a antibody domain (eg, V or C domain, eg, VH, VL, Vκ, Vλ, CH, CH1, CH2, CH3, hige, Cκ or Cλ domain). In an example, the TD is a TD comprised by any one of SEQ ID NOs: 1-9. In an example, the TD is a TD comprising SEQ ID NO: 10 or 126. In an example, the TD is encoded by SEQ ID NO: 124 or 125. In an example, the amino acid sequence of each TD is SEQ ID NO: 10 or 126 or is at least 80, 85, 90, 95, 96m 97, 98 or 99% identical to the SEQ ID NO: 10 or 126. In an example, the TD is a TD comprising SEQ ID NO: 120 or 123. In an example, the TD is encoded by SEQ ID NO: 116 or 119. In an example, the amino acid sequence of each TD is SEQ ID NO: 120 or 123 or is at least 80, 85, 90, 95, 96m 97, 98 or 99% identical to the SEQ ID NO: 120 or 123. Optionally, the domain or peptide comprised by the engineered polypeptide or monomer comprises an amino acid selected from SEQ ID NOs: 51-82. The sequences in this paragraph are those sequences disclosed in US11,453,726, which sequences are incorporated herein for use with the present invention. The homologue, orthologue or equivalent has self-associating multimerisation or tetramerization function. Homologue: A gene, nucleotide or protein sequence related to a second gene, nucleotide or protein sequence by descent from a common ancestral DNA or protein sequence. The term, homologue, may apply to the relationship between genes separated by the event of or to the relationship between genes separated by the event of genetic duplication. Orthologue: Orthologues are genes, nucleotide or protein sequences in different species that evolved from a common ancestral gene, nucleotide or protein sequence by speciation. Normally, orthologues retain the same function in the course of evolution. In an embodiment, each polypeptide or monomer comprises one or more VH, VL or VH/VL binding sites of an antibody selected from ReoPro™; Abciximab; Rituxan™; Rituximab; Zenapax™; Daclizumab; Simulect™; Basiliximab; Synagis™; Palivizumab; Remicade™; Infliximab; Herceptin™; Trastuzumab; Mylotarg™; Gemtuzumab; Campath™; Alemtuzumab; Zevalin™; Ibritumomab; Humira™; Adalimumab; Xolair™; Omalizumab; Bexxar™; Tositumomab; Raptiva™; Efalizumab; Erbitux™; Cetuximab; Avastin™; Bevacizumab; Tysabri™; Natalizumab; Actemra™; Tocilizumab; Vectibix™; Panitumumab; Lucentis™; Ranibizumab; Soliris™; Eculizumab; Cimzia™; Certolizumab; Simponi™; Golimumab, Ilaris™; Canakinumab; Stelara™; Ustekinumab; Arzerra™; Ofatumumab; Prolia™; Denosumab; Numax™; Motavizumab; ABThrax™; Raxibacumab; Benlysta™; Belimumab; Yervoy™; Ipilimumab; Adcetris™; Brentuximab; Vedotin™; Perjeta™; Pertuzumab; Kadcyla™; Ado-trastuzumab; Gazyva™ and Obinutuzumab. In an alternative, (eg, for treating or preventing a cancer in a human) each polypeptide or monomer comprise one or more VH, VL or VH/VL binding sites of an antibody selected from ipilimumab (or YERVOY^TM), tremelimumab, nivolumab (or OPDIVO^TM), pembrolizumab (or KEYTRUDA^TM), pidilizumab, BMS-936559, durvalumab and atezolizumab. In an example, the multimer or tetramer comprises 4 copies of the antigen binding site of a first antibody selected from the group consisting of ipilimumab (or YERVOY^TM), tremelimumab, nivolumab (or OPDIVO^TM), pembrolizumab (or KEYTRUDA^TM), pidilizumab, BMS-936559, durvalumab and atezolizumab and optionally 4 copies of the antigen binding site of a second antibody selected from said group, wherein the first and second antibodies are different. For example, the first antibody is ipilimumab (or YERVOY^TM) and optionally the second antibody is nivolumab (or OPDIVO^TM) or pembrolizumab (or KEYTRUDA^TM). This is useful for treating or preventing a cancer in a human. In an example, the multimer or tetramer comprises 4 copies of the antigen binding site of Avastin. In an example, the multimer or tetramer comprises 4 copies of the antigen binding site of Humira. In an example, the multimer or tetramer comprises 4 copies of the antigen binding site of Erbitux. In an example, the multimer or tetramer comprises 4 copies of the antigen binding site of Actemra™. In an example, the multimer or tetramer comprises 4 copies of the antigen binding site of sarilumab. In an example, the multimer or tetramer comprises 4 copies of the antigen binding site of dupilumab. In an example, the multimer or tetramer comprises 4 copies of the antigen binding site of alirocumab or evolocumab. In an example, the multimer or tetramer comprises 4 copies of the antigen binding site of In an example, the multimer or tetramer comprises 4 copies of the antigen binding site of Remicade. In an example, the multimer or tetramer comprises 4 copies of the antigen binding site of Lucentis. In an example, the multimer or tetramer comprises 4 copies of the antigen binding site of Eylea™. Such tetramers are useful for administering to a human to treat or prevent a cancer. Such tetramers are useful for administering to a human to treat or prevent an ocular condition (eg, wet AMD or diabetic retinopathy, eg, when the binding site is an Avastin, Lucentis or Eylea site). Such tetramers are useful for administering to a human to treat or prevent angiogenesis. In an example, the multimer or tetramer comprises 4 copies of insulin. In an example, the multimer or tetramer comprises 4 copies of GLP-1. In an example, the multimer or tetramer comprises 4 copies of GIP. In an example, the multimer or tetramer comprises 4 copies of Exendin-4. In an example, the multimer or tetramer comprises 4 copies of insulin and 4 copies of GLP-1. In an example, the multimer or tetramer comprises 4 copies of insulin and 4 copies of GIP. In an example, the multimer or tetramer comprises 4 copies of insulin and 4 copies of Exendin-4. In an example, the multimer or tetramer comprises 4 copies of GLP-1 and 4 copies of Exendin-4. Such tetramers are useful for administering to a human to treat or prevent diabetes (eg, Type II diabetes) or obesity. Immunoglobulin variable domains Antibody variable domains are known in the art and available from a wide variety of sources. Databases of sequences of antibody variable domains exist, such as IMGT and Kabat, and variable domains can be produced by cloning and expression of natural sequences, or synthesis of artificial nucleic acids according to established techniques. Methods for the construction of bacteriophage antibody display libraries and lambda phage expression libraries are well known in the art (McCafferty et al. (1990) Nature, 348: 552; Kang et al. (1991) Proc. Natl. Acad. Sci. USA., 88: 4363; Clackson et al. (1991) Nature, 352: 624; Lowman et al. (1991) Biochemistry, 30: 10832; Burton et al. (1991) Proc. Natl. Acad Sci USA., 88: 10134; Hoogenboom et al. (1991) Nucleic Acids Res., 19: 4133; Chang et al. (1991) J Immunol., 147: 3610; Breitling et al. (1991) Gene, 104: 147; Marks et al. (1991) supra; Barbas et al. (1992) supra; Hawkins and Winter (1992) J Immunol., 22: 867; Marks et al., 1992, J Bioi. Chem., 267: 16007; Lerner et al. (1992) Science, 258: 1313, incorporated herein by reference). One particularly advantageous approach has been the use of scFv phage-libraries (Huston et al., 1988, Proc. Natl. Acad. Sci U.S.A., 85: 5879-5883; Chaudhary et al. (1990) Proc. Natl. Acad. Sci U.S.A., 87: 1066-1070; McCafferty et al. (1990) supra; Clackson et al. (1991) Nature, 352: 624; Marks et al. (1991) J Mol. Bioi., 222: 581; Chiswell et al. (1992) Trends Biotech., 10: 80; Marks et al. (1992) J Bioi. Chem., 267). Various embodiments of scFv libraries displayed on bacteriophage coat proteins have been described. Refinements of phage display approaches are also known, for example as described in W096/06213 and W092/01047 (Medical Research Council et al.) and W097/08320 (Morphosys), which are incorporated herein by reference. Such techniques can be adapted for the production of multimeric immunoglobulins by the fusion of NHR2 multimerisation domains to the antibody variable domains Immunoglobulin constant domains An immunoglobulin constant domain, as referred to herein, is preferably an antibody constant domain. Constant domains do vary in sequence between antibody subtypes; preferably, the constant domains are IgG constant domains. Preferably, the constant domains are CH1 constant domains. Antibody constant domains are well known in the art and available from a number of sources and databases, including the IMGT and Kabat databases. The fusion of antibody constant domains to immunoglobulin variable domains is also known in the art, for example in the construction of engineered Fab antibody fragments. Linkers Flexible linkers can be used to connect TCR variable domain – Ig constant domain to the NHR2 multimerisation domain. This allows the TCR domains and the multimerisation domain to function without steric hindrance from each other or other molecules in the multimeric complex. Suitable linkers comprise, for example, glycine repeats, glycine-alanine repeats, Gly(4)Ser linkers, or flexible polypeptide linkers as set forth in Reddy Chichili et al., 2012 Protein Science 22:153-167. Immunoglobulin Hinge The Ig Hinge, herein preferably an antibody hinge, is the polypeptide sequence which links antibody constant regions in a natural antibody. This therefore provides for natural dimerization of molecules which include an antibody constant domain. It is present, for example, in a F(ab)2 antibody fragment, as well as whole antibodies such as IgG. This region comprises two natural interchain disulphide bonds, which connect the two CH1 constant domains together. The multimerisation domain (SAM), in one embodiment, may be attached to the Ig constant domain or to the hinge. If a hinge is present, the multimerisation domain will form a TRC octamer, comprising four dimers of TCR variable-Ig Constant domains joined at a hinge region. Without the hinge region, the multimerisation domain will lead to the formation of a tetramer. Preferably, the multimerisation domain is attached to the C-terminal end of the constant domain or the hinge region. A hinge may be a hinge devoid of a core region, such as any hinge disclosed in US2022162285, the disclosure of which is incorporated herein by reference for use with the present invention. Uses of TCR Multimers Multimeric TCR proteins according to the invention are useful in any application in which soluble TCR proteins are indicated. Particular advantages of the TCR proteins of the invention include increased avidity for the selected target, and/or the ability to bind a plurality of targets. Thus, in one aspect, the multivalent heterodimeric soluble TCR protein molecules of the invention can be used for selectively inhibiting immune responses, for example suppression of an autoimmune response. The multivalent, for example tetravalent, nature of these soluble protein molecules gives it exquisite sensitivity and binding affinity to compete antigen- specific interactions between T cells and antigen presenting cells. This kind of neutralization effect can be therapeutically beneficial in autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, psoriasis, inflammatory bowel diseases, graves disease, vasculitis and type 1 diabetes. Similarly, the tetravalent heterodimeric soluble TCR protein molecules can be used to prevent tissue transplant rejection by selectively suppressing T cell recognition of specific transplantation antigen and self antigens binding to target molecule and thus inhibiting cell- to-cell interaction. In another aspect of the invention, the tetravalent heterodimeric soluble TCR protein molecules can be used in clinical studies such as toxicity, infectious disease studies, neurological studies, behavior and cognition studies, reproduction, genetics and xenotransplantation studies. The tetravalent heterodimeric soluble TCR protein molecules with enhanced sensitivity for cognate pMHC can be used for the purpose of diagnostics using biological samples obtained directly from human patients. The enhanced sensitivity of the tetravalent heterodimeric soluble TCRs allows detection of potential disease-associated peptides displayed on MHC, which are naturally found to be expressed at low density. These molecules can also be used for patient stratification for enrolling patient onto relevant clinical trials. In another aspect of the invention, octavalent heterodimeric soluble TCR protein molecules can be used in pharmaceutical preparations for the treatment of various diseases. In another related aspect to this invention, octavalent heterodimeric soluble TCR protein molecules can be used as a probe for tumour molecular imaging or prepared as a therapeutic protein. There is provided:- A composition or multimer as described herein for treating or preventing a disease or condition in a subject. A method of for treating or preventing a disease or condition in a subject, the method comprising administering the composition or multimer to the subject. The subject may be a human or animal. For example the subject is a human. The subject may be a plant or fungus. Diseases & Conditions A disease or condition in a human or animal herein may be selected from any of the following. (a) A neurodegenerative disease or condition; (b) A brain disease or condition; (c) A CNS disease or condition; (d) Memory loss or impairment; (e) A heart or cardiovascular disease or condition, eg, heart attack, stroke or atrial fibrillation; (f) A liver disease or condition; (g) A kidney disease or condition, eg, chronic kidney disease (CKD); (h) A pancreas disease or condition; (i) A lung disease or condition, eg, cystic fibrosis or COPD; (j) A gastrointestinal disease or condition; (k) A throat or oral cavity disease or condition; (l) An ocular disease or condition; (m) A genital disease or condition, eg, a vaginal, labial, penile or scrotal disease or condition; (n) A sexually-transmissible disease or condition, eg, gonorrhea, HIV infection, syphilis or Chlamydia infection; (o) An ear disease or condition; (p) A skin disease or condition; (q) A heart disease or condition; (r) A nasal disease or condition (s) A haematological disease or condition, eg, anaemia, eg, anaemia of chronic disease or cancer; (t) A viral infection; (u) A pathogenic bacterial infection; (v) A cancer; (w) An autoimmune disease or condition, eg, SLE; (x) An inflammatory disease or condition, eg, rheumatoid arthritis, psoriasis, eczema, asthma, ulcerative colitis, colitis, Crohn’s disease or IBD; (y) Autism; (z) ADHD; (aa) Bipolar disorder; (bb) ALS [Amyotrophic Lateral Sclerosis]; (cc) Osteoarthritis; (dd) A congenital or development defect or condition; (ee) Miscarriage; (ff) A blood clotting condition; (gg) Bronchitis; (hh) Dry or wet AMD; (ii) Neovascularisation (eg, of a tumour or in the eye); (jj) Common cold; (kk) Epilepsy; (ll) Fibrosis, eg, liver or lung fibrosis; (mm) A fungal disease or condition, eg, thrush; (nn) A metabolic disease or condition, eg, obesity, anorexia, diabetes, Type I or Type II diabetes. (oo) Ulcer(s), eg, gastric ulceration or skin ulceration; (pp) Dry skin; (qq) Sjogren’s syndrome; (rr) Cytokine storm; (ss) Deafness, hearing loss or impairment; (tt) Slow or fast metabolism (ie, slower or faster than average for the weight, sex and age of the subject); (uu) Conception disorder, eg, infertility or low fertility; (vv) Jaundice; (ww) Skin rash; (xx) Kawasaki Disease; (yy) Lyme Disease; (zz) An allergy, eg, a nut, grass, pollen, dust mite, cat or dog fur or dander allergy; (aaa) Malaria, typhoid fever, tuberculosis or cholera; (bbb) Depression; (ccc) Mental retardation; (ddd) Microcephaly; (eee) Malnutrition; (fff) Conjunctivitis; (ggg) Pneumonia; (hhh) Pulmonary embolism; (iii) Pulmonary hypertension; (jjj) A bone disorder; (kkk) Sepsis or septic shock; (lll) Sinusitus; (mmm) Stress (eg, occupational stress); (nnn) Thalassaemia, anaemia, von Willebrand Disease, or haemophilia; (ooo) Shingles or cold sore; (ppp) Menstruation; (qqq) Low sperm count. NEURODEGENERATIVE OR CNS DISEASES OR CONDITIONS FOR TREATMENT OR PREVENTION BY THE METHOD In an example, the neurodegenerative or CNS disease or condition is selected from the group consisting of Alzheimer disease , geriopsychosis, Down syndrome, Parkinson's disease, Creutzfeldt-jakob disease, diabetic neuropathy, Parkinson syndrome, Huntington's disease, Machado-Joseph disease, amyotrophic lateral sclerosis, diabetic neuropathy, and Creutzfeldt Creutzfeldt- Jakob disease. For example, the disease is Alzheimer disease. For example, the disease is Parkinson syndrome. In an example, wherein the method of the invention is practised on a human or animal subject for treating a CNS or neurodegenerative disease or condition, the method causes downregulation of Treg cells in the subject, thereby promoting entry of systemic monocyte-derived macrophages and/or Treg cells across the choroid plexus into the brain of the subject, whereby the disease or condition (eg, Alzheimer’s disease) is treated, prevented or progression thereof is reduced. In an embodiment the method causes an increase of IFN-gamma in the CNS system (eg, in the brain and/or CSF) of the subject. In an example, the method restores nerve fibre and//or reduces the progression of nerve fibre damage. In an example, the method restores nerve myelin and//or reduces the progression of nerve myelin damage. In an example, the method of the invention treats or prevents a disease or condition disclosed in WO2015136541 and/or the method can be used with any method disclosed in WO2015136541 (the disclosure of this document is incorporated by reference herein in its entirety, eg, for providing disclosure of such methods, diseases, conditions and potential therapeutic agents that can be administered to the subject for effecting treatement and/or prevention of CNS and neurodegenerative diseases and conditions, eg, agents such as immune checkpoint inhibitors, eg, anti-PD-1, anti-PD-L1, anti-TIM3 or other antibodies disclosed therein). CANCERS FOR TREATMENT OR PREVENTION BY THE METHOD Cancers that may be treated include tumours that are not vascularized, or not substantially vascularized, as well as vascularized tumours. The cancers may comprise non-solid tumours (such as haematological tumours, for example, leukaemias and lymphomas) or may comprise solid tumours. Types of cancers to be treated with the invention include, but are not limited to, carcinoma, blastoma, and sarcoma, and certain leukaemia or lymphoid malignancies, benign and malignant tumours, and malignancies e.g., sarcomas, carcinomas, and melanomas. Adult tumours/cancers and paediatric tumours/cancers are also included. Haematologic cancers are cancers of the blood or bone marrow. Examples of haematological (or haematogenous) cancers include leukaemias, including acute leukaemias (such as acute lymphocytic leukaemia, acute myelocytic leukaemia, acute myelogenous leukaemia and myeloblasts, promyeiocytic, myelomonocytic, monocytic and erythroleukaemia), chronic leukaemias (such as chronic myelocytic (granulocytic) leukaemia, chronic myelogenous leukaemia, and chronic lymphocytic leukaemia), polycythemia vera, lymphoma, Hodgkin's disease, non-Hodgkin's lymphoma (indolent and high grade forms), multiple myeloma, Waldenstrom's macroglobulinemia, heavy chain disease, myeiodysplastic syndrome, hairy cell leukaemia and myelodysplasia. Solid tumours are abnormal masses of tissue that usually do not contain cysts or liquid areas. Solid tumours can be benign or malignant. Different types of solid tumours are named for the type of cells that form them (such as sarcomas, carcinomas, and lymphomas). Examples of solid tumours, such as sarcomas and carcinomas, include fibrosarcoma, myxosarcoma, liposarcoma, chondrosarcoma, osteosarcoma, and other sarcomas, synovioma, mesothelioma, Ewing's tumour, leiomyosarcoma, rhabdomyosarcoma, colon carcinoma, lymphoid malignancy, pancreatic cancer, breast cancer, lung cancers, ovarian cancer, prostate cancer, hepatocellular carcinoma, squamous eel! carcinoma, basal cell carcinoma, adenocarcinoma, sweat gland carcinoma, medullary thyroid carcinoma, papillary thyroid carcinoma, pheochromocytomas sebaceous gland carcinoma, papillary carcinoma, papillary adenocarcinomas, medullary carcinoma, bronchogenic carcinoma, renal cell carcinoma, hepatoma, bile duct carcinoma, choriocarcinoma, Wilms' tumour, cervical cancer, testicular tumour, seminoma, bladder carcinoma, melanoma, and CNS tumours (such as a glioma (such as brainstem glioma and mixed gliomas), glioblastoma (also known as glioblastoma multiforme) astrocytoma, CNS lymphoma, germinoma, medu!loblastoma, Schwannoma craniopharyogioma, ependymoma, pineaioma, hemangioblastoma, acoustic neuroma, oligodendroglioma, menangioma, neuroblastoma, retinoblastoma and brain metastases). AUTOIMMUNE DISEASES FOR TREATMENT OR PREVENTION BY THE METHOD • Acute Disseminated Encephalomyelitis (ADEM) • Acute necrotizing hemorrhagic leukoencephalitis • Addison’s disease • Agammaglobulinemia • Alopecia areata • Amyloidosis • Ankylosing spondylitis • Anti-GBM/Anti-TBM nephritis • Antiphospholipid syndrome (APS) • Autoimmune angioedema • Autoimmune aplastic anemia • Autoimmune dysautonomia • Autoimmune hepatitis • Autoimmune hyperlipidemia • Autoimmune immunodeficiency • Autoimmune inner ear disease (AIED) • Autoimmune myocarditis • Autoimmune oophoritis • Autoimmune pancreatitis • Autoimmune retinopathy • Autoimmune thrombocytopenic purpura (ATP) • Autoimmune thyroid disease • Autoimmune urticaria • Axonal & neuronal neuropathies • Balo disease • Behcet’s disease • Bullous pemphigoid • Cardiomyopathy • Castleman disease • Celiac disease • Chagas disease • Chronic fatigue syndrome • Chronic inflammatory demyelinating polyneuropathy (CIDP) • Chronic recurrent multifocal ostomyelitis (CRMO) • Churg-Strauss syndrome • Cicatricial pemphigoid/benign mucosal pemphigoid • Crohn’s disease • Cogans syndrome • Cold agglutinin disease • Congenital heart block • Coxsackie myocarditis • CREST disease • Essential mixed cryoglobulinemia • Demyelinating neuropathies • Dermatitis herpetiformis • Dermatomyositis • Devic’s disease (neuromyelitis optica) • Discoid lupus • Dressler’s syndrome • Endometriosis • Eosinophilic esophagitis • Eosinophilic fasciitis • Erythema nodosum • Experimental allergic encephalomyelitis • Evans syndrome • Fibromyalgia • Fibrosing alveolitis • Giant cell arteritis (temporal arteritis) • Giant cell myocarditis • Glomerulonephritis • Goodpasture’s syndrome • Granulomatosis with Polyangiitis (GPA) (formerly called Wegener’s Granulomatosis) • Graves’ disease • Guillain-Barre syndrome • Hashimoto’s encephalitis • Hashimoto’s thyroiditis • Hemolytic anemia • Henoch-Schonlein purpura • Herpes gestationis • Hypogammaglobulinemia • Idiopathic thrombocytopenic purpura (ITP) • IgA nephropathy • IgG4-related sclerosing disease • Immunoregulatory lipoproteins • Inclusion body myositis • Interstitial cystitis • Juvenile arthritis • Juvenile diabetes (Type 1 diabetes) • Juvenile myositis • Kawasaki syndrome • Lambert-Eaton syndrome • Leukocytoclastic vasculitis • Lichen planus • Lichen sclerosus • Ligneous conjunctivitis • Linear IgA disease (LAD) • Lupus (SLE) • Lyme disease, chronic • Meniere’s disease • Microscopic polyangiitis • Mixed connective tissue disease (MCTD) • Mooren’s ulcer • Mucha-Habermann disease • Multiple sclerosis • Myasthenia gravis • Myositis • Narcolepsy • Neuromyelitis optica (Devic’s) • Neutropenia • Ocular cicatricial pemphigoid • Optic neuritis • Palindromic rheumatism • PANDAS (Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcus) • Paraneoplastic cerebellar degeneration • Paroxysmal nocturnal hemoglobinuria (PNH) • Parry Romberg syndrome • Parsonnage-Turner syndrome • Pars planitis (peripheral uveitis) • Pemphigus • Peripheral neuropathy • Perivenous encephalomyelitis • Pernicious anemia • POEMS syndrome • Polyarteritis nodosa • Type I, II, & III autoimmune polyglandular syndromes • Polymyalgia rheumatica • Polymyositis • Postmyocardial infarction syndrome • Postpericardiotomy syndrome • Progesterone dermatitis • Primary biliary cirrhosis • Primary sclerosing cholangitis • Psoriasis • Psoriatic arthritis • Idiopathic pulmonary fibrosis • Pyoderma gangrenosum • Pure red cell aplasia • Raynauds phenomenon • Reactive Arthritis • Reflex sympathetic dystrophy • Reiter’s syndrome • Relapsing polychondritis • Restless legs syndrome • Retroperitoneal fibrosis • Rheumatic fever • Rheumatoid arthritis • Sarcoidosis • Schmidt syndrome • Scleritis • Scleroderma • Sjogren’s syndrome • Sperm & testicular autoimmunity • Stiff person syndrome • Subacute bacterial endocarditis (SBE) • Susac’s syndrome • Sympathetic ophthalmia • Takayasu’s arteritis • Temporal arteritis/Giant cell arteritis • Thrombocytopenic purpura (TTP) • Tolosa-Hunt syndrome • Transverse myelitis • Type 1 diabetes • Ulcerative colitis • Undifferentiated connective tissue disease (UCTD) • Uveitis • Vasculitis • Vesiculobullous dermatosis • Vitiligo • Wegener’s granulomatosis (now termed Granulomatosis with Polyangiitis (GPA). INFLAMMATORY DISEASES FOR TREATMENT OR PREVENTION BY THE METHOD • Alzheimer's • ankylosing spondylitis • arthritis (osteoarthritis, rheumatoid arthritis (RA), psoriatic arthritis) • asthma • atherosclerosis • Crohn's disease • colitis • dermatitis • diverticulitis • fibromyalgia • hepatitis • irritable bowel syndrome (IBS) • systemic lupus erythematous (SLE) • nephritis • Parkinson's disease • ulcerative colitis. There is further provided the following generally-applicable numbered Options (which may, for example, be combined with any Configuration herein). 1. Optionally, the locus comprises (in 5' to 3' direction) (a) the first nucleotide sequence; (b) an antibody chain intronic enhancer; and (b) an antibody chain constant region comprising the second nucleotide sequence and encoding an antibody chain C domain; wherein the locus is operable to express an antibody chain comprising EM1, SAM and the C domain, wherein EM1 is C-terminal to the SAM and C domain. 2. Optionally, the locus is a kappa light chain locus and the enhancer is Εiκ. 3. Optionally, the cell is a mouse cell, locus is a lambda light chain locus and the enhancer is a mouse lambda 2-4 or 4-10 enhancer. 4. Optionally, (a) EM1 is a rearranged Vk domain and the C domain is a Cκ; (b) EM1 is a rearranged Vk domain and the C domain is a Cλ; (c) EM1 is a rearranged Vλ domain and the C domain is a Cκ; or (d) EM1 is a rearranged Vλ domain and the C domain is a Cλ. 5. Optionally, EM1 is a rearranged VH and the C domain is a CH1 or CH2 domain (eg, wherein the CH2 is comprised by an antibody Fc). 6. Optionally, the locus comprises (in 5' to 3' direction) (a) a promoter operable for promoting transcription of the first nucleotide sequence; (b) a nucleotide sequence encoding a signal peptide for EM1 secretion; (c) said intronic enhancer; (d) (i) said second nucleotide sequence; and a sequence encoding the C domain; or (ii) a sequence encoding the C domain and said second nucleotide sequence, Wherein the locus is operable to express RNA transcripts encoding (in N- to C-terminal direction) said signal peptide sequence fused to the amino acid sequence of EM1 and SAM. 7. Optionally, the cell is an ES, iPS, hybridoma or B-cell. 8. Optionally, EM1 is a rearranged V domain, eg, VH, or a kappa or lambda V domain. 9. Optionally, EM1 has a binding specificity for a first predetermined antigen or a first epitope, wherein the locus is operable to express a polypeptide that comprises a V domain that comprises said specificity. 10. Optionally, EM1 is a rearranged V domain and the cell comprises a second Ig locus that is operable to express a second polypeptide, wherein the second polypeptide comprises a second rearranged V domain that forms a binding site with EM1, wherein the binding site is capable of specifically binding to a predetermined antigen or epitope. 11. Optionally, the predetermined antigen is any antigen disclosed herein. 12. The cell of claim 11, wherein (a) EM1 is a VL domain and the second V domains is a VH domain; or (b) EM1 is a VH domain and the second V domains is a VL domain. 13. Optionally, the second nucleotide sequence is at least 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, 450 or 500 kb 3' of the enhancer. This is useful to distance the second nucleotide sequence from any effects of the enhancer or to minimise mutation of the SAM. 14. Optionally, the locus comprises a second enhancer that is 3' of the constant region. 15. Optionally, the cell is a non-human mammal, mouse, rat or rodent cell. 16. Optionally, said constant region is at an endogenous antibody locus of the cell. 17. Optionally, said endogenous locus is an endogenous kappa chain locus. 18. Optionally, said endogenous locus is an endogenous heavy chain locus. 19. Optionally, the locus is comprised by a transgene that is comprised by the genome of the cell at a position outside an endogenous antibody locus. 20. Optionally, the cell is homozygous for said locus. 21. Optionally, the cell genome comprises a second antibody locus, wherein the second locus is an unrearranged antibody heavy chain locus comprising (in 5' to 3' direction) (a) one or more VH gene segments; (b) one or more DH gene segments; (c) one or more JH gene segments; and (d) a heavy chain constant region encoding one or more CH domains; Wherein the heavy chain locus is operable to express a plurality of heavy chains, optionally comprising a plurality of antigen specificities or affinities, each said heavy chain being capable of pairing with an antibody chain encoded by the first locus to produce paired chains that comprise an antigen binding site. 22. Optionally, the cell comprises one or more further antibody loci, each further locus being capable of expressing a light chain, each said light chain being capable of pairing with a polyeptide encoded by the first locus to produce paired chains that comprise an antigen binding site. 23. Optionally, each variable region or gene segment is human gene segment. 24. Optionally, the cell is a mouse cell and each constant region is a mouse, rat or human constant region. 25. Optionally, each said enhancer is an endogenous enhancer of the cell. 26. The cell of any preceding claim, wherein the cell is a mouse cell and each said enhancer is a mouse enhancer. 27. Optionally, (a) the cell is a mouse cell, EM1 is a human variable region, the intronic enhancer is a mouse intronic enhancer at an endogenous antibody locus of the cell and said constant region is a mouse, rat or human constant region; or (b) the cell is a rat cell, EM1 is a human variable region, the intronic enhancer is a rat intronic enhancer at an endogenous antibody locus of the cell and said constant region is a mouse, rat or human constant region. 28. Optionally, EM1 is a rearranged variable region that is encoded by a rearrangement of (a) human IGLV3-21 and IGLJ3 and optionally operably connected to human germline IGLV3-21 promoter and/or signal peptide-encoding nucleotide sequence; (b) human IGK1-39 and IGKJ1 or 5 and optionally operably connected to human germline IGKV1-39 promoter and/or signal peptide-encoding nucleotide sequence; (c) human IGK3-20 and IGKJ1 or 5 and optionally operably connected to human germline IGKV3-20 promoter and/or signal peptide-encoding nucleotide sequence; (d) a human VpreB and J\5 and optionally operably connected to human germline VpreB promoter and/or signal peptide-encoding nucleotide sequence. 29. A transgenic non-human vertebrate comprising a plurality of cells according to any preceding Option. 30. Optionally, said vertebrate comprises a first locus (and optionally the second locus) as defined in any one of Options 1 to 28. 31. Optionally, the vertebrate is a chimaera of said cells and a plurality of other cells that do not comprise the first locus, wherein the germline of the vertebrate does not comprise a said first locus. CONCEPTS The invention further provides the following Concepts. 1. A non-human vertebrate (eg, a mouse or a rat), wherein an immunoglobulin (Ig) locus of the genome of the vertebrate comprises a first nucleotide sequence for producing a first effector moiety (EM1) and a second nucleotide sequence encoding a self-associating multimerisation domain (SAM), wherein the locus is capable of producing copies of a polypeptide comprising EM1 and SAM, wherein the SAM domains of the polypeptides associate together to produce a polypeptide multimer comprising a plurality of EM1 moieties. 2. The vertebrate of Concept 1, wherein the first nucleotide sequence is an Ig locus variable region. 3. The vertebrate of Concept 2, wherein the variable region is an unrearranged variable region,that is capable of rearrangement to produce a nucleotide sequence encoding EM1. 4. The vertebrate of any preceding Concept, wherein the locus comprises, in 5’ to 3’ direction, the first nucleotide sequence and the second nucleotide sequence; or the locus comprises, in 5’ to 3’ direction, the second nucleotide sequence and the first nucleotide sequence. 5. The vertebrate of any preceding Concept, wherein the locus comprises a third nucleotide sequence for producing a second effector moiety (EM2), wherein the third sequence is 5’ of the first sequence; the third sequence is 3’ of the first sequence; or the third sequence is between the first and second sequences. 6. The vertebrate of Concept 5, wherein EM1 is capable of binding to a first antigen or epitope; EM2 is capable of binding to a second antigen or epitope; and a) The first and second antigens are identical; b) The first and second epitopes are identical; c) The first and second antigens are different from each other; d) The first and second epitopes are different from each other. 7. The vertebrate of Concept 5 or 6, wherein the first nucleotide sequence is an unrearranged Ig variable region and the third nucleotide sequence is a rearranged Ig variable region. 8. The vertebrate of any preceding Concept, wherein the locus is an antibody locus or a T- cell receptor locus. 9. The vertebrate of any preceding Concept, wherein each said effector moiety comprises a) a protein domain; or b) a peptide; optionally wherein each said effector moiety comprises an Ig variable domain (eg, an antibody VH or VL; or a TCR Vα, Vβ, Vγ or Vδ) or an epitope binding domain. 10. The vertebrate of Concept 9, wherein each said effector moiety is an antibody single variable domain. 11. The vertebrate of any preceding Concept, wherein SAM is a tetramerization domain (TD), optionally a p53, TTR, TPR, BCR, L27 TD. 12. The vertebrate of any preceding Concept, wherein the second nucleotide sequence is comprised by an Ig constant region of the locus (optionally wherein the locus is an antibody locus and the second nucleotide sequence is comprised by a gamma constant region). 13. The vertebrate of any preceding Concept (optionally according to Concept 3), wherein the locus is an antibody locus and comprises, in 5’ to 3’ direction, a) a first constant region of a first isotype (optionally a mu isotype); and b) a second constant region of a second isotype (optionally a non-mu isotype, such as a gamma isotype), wherein the second constant region comprises said second nucleotide sequence; wherein the locus is capable of isotype switching from the first isotype to the second isotype wherein the locus is capable of expressing polypeptides of the second isotype wherein each polypeptide comprises EM1 and a said SAM, optionally wherein the second constant region comprises a third nucleotide sequence that encodes a second effector moiety (EM2) wherein EM1 and EM2 are capable of binding to different antigens. 14. The vertebrate of Concept 13, wherein the first constant region is devoid of a SAM. 15. The vertebrate of Concept 13 or 14, wherein the second constant region is devoid of a nucleotide sequence encoding a CH1. 16. The vertebrate of Concept 13, wherein the locus is capable of producing a polypeptide selected from a polypeptide comprising, in N- to C-terminal direction, a) EM1 and SAM; b) SAM and EM1; c) EM1, EM2 and SAM; d) SAM, EM1 and EM2 e) EM1, SAM and EM2; f) EM1, EM2, EM3 and SAM; g) SAM, EM1, EM2 and EM3; h) EM1,EM2 SAM and EM3; i) EM1 SAM, EM2 and EM3; j) EM1, EM2, EM3, EM4 and SAM; k) SAM, EM1, EM2, EM3 and EM4; l) EM1, EM2, EM3 SAM and EM4; m) EM1 SAM, EM2, EM3 and EM4; n) EM1, EM2 SAM, EM3 and EM4; o) EM1, EM2, EM3, EM4, EM5 and SAM; p) SAM, EM1, EM2, EM3, EM4 and EM5; q) EM1, EM2, EM3 SAM, EM4 and EM5; r) EM1, EM2 SAM, EM3, EM4 and EM5; s) EM1, EM2, EM3 SAM, EM4 and EM5; t) EM1, EM2, SAM, EM3, EM4 and EM5; and u) EM1, EM2, EM3, SAM, EM4, EM5 and EM6; Wherein each EM is an effector moiety (optionally an Ig variable domain or peptide); and optionally wherein EM1 is capable of binding to a first antigen or epitope; EM2 is capable of binding to a second antigen or epitope; and a) The first and second antigens are identical; b) The first and second epitopes are identical; c) The first and second antigens are different from each other; d) The first and second epitopes are different from each other. A non-human vertebrate or a non-human vertebrate cell that comprises a rearranged Ig variable region (eg, an VH or VL region) encoding a V domain, wherein the variable region is ectopically positioned in the genome of the vertebrate or cell, wherein the vertebrate or cell expresses polypeptides each comprising a SAM and a said V domain, wherein the V domains of the polypeptides comprise most commonly a CDR3 length of 11 to 16 (eg, 15) amino acids. A non-human vertebrate (eg, a mouse) according to any one of Concepts 1-19 or a non- human vertebrate cell (eg, a mouse cell) whose genome comprises a locus for expression of antibody heavy chains, the locus comprising a) an unrearranged human variable region comprising human variable region gene segments for expression of a repertoire of human variable domains (EM1 moieties); b) an endogenous mu constant region for expression of IgM antibody heavy chains comprising endogenous mu heavy chain constant domains and human variable domains; and c) (i) a humanised non-mu constant region downstream of the mu constant region for expression of non-mu antibody heavy chains comprising human non-mu constant domains and human variable domains, wherein the non-mu constant region comprises a nucleotide sequence encoding a SAM; or (ii) a non-human vertebrate (eg, mouse) non-mu constant region downstream of the mu constant region for expression of non-mu antibody heavy chains comprising non-human vertebrate (eg, mouse) non-mu constant domains and human variable domains, wherein the non-mu constant region comprises a nucleotide sequence encoding a SAM; wherein the unrearranged variable region is provided as a targeted insertion of the human variable region gene segments upstream of the endogenous mu constant region in an endogenous IgH locus such that the variable region gene segments are able to recombine for expression and selection in the context of an endogenous mu constant region. A non-human vertebrate (eg, a mouse or a rat) according to any one of Concepts 1-19 or a non-human vertebrate cell whose genome comprises an antibody heavy chain locus comprising (in 5′ to 3′ direction) a variable region, a first switch, an endogenous mu constant region, a second switch and a human non-mu (eg, gamma) constant region, wherein the non-mu constant region comprises a nucleotide sequence encoding a SAM; wherein the heavy chain locus of each cell is capable of undergoing switching from IgM to the non-mu (eg, IgG) isotype for the production of non-mu heavy chains comprising a rearranged V domain and a SAM. A cell or a plurality of cells, wherein each cell is a) an isolated lymphocytic cell (optionally a B- or T-cell) obtainable from a vertebrate of any preceding Concept, each comprises a locus as defined in any preceding Concept for producing polypeptides comprising EM1 and a SAM.; or b) an embryonic stem cell or pluripotent stem cell that can develop into a vertebrate according to any preceding Concept. The cell(s) of Concept 20(a), wherein the germline of the vertebrate genome comprises a locus according to Concept 13 when dependent from Concept 3 and each said cell comprises an isotype switched locus obtainable by switching the locus recited in Concept 13 to the second isotype, wherein the switched locus comprises a rearranged variable region encoding EM1 and produced by rearrangement of the variable region recited in Concept 3, wherein each said cell produces a respective polypeptide comprising an EM1 and a SAM. The cell(s) of Concept 21, wherein each said cell is capable of secreting a multimer of its respective polypeptide, wherein copies of the polypeptide are associated by SAMs. A lymphocyte cell obtainable from the vertebrate of any one of Concepts 1-19 and comprising a locus for expressing a polypeptide, wherein the locus comprises (in 5' to 3' direction) (a) an unrearranged Ig variable region comprising at least one V gene segment, optionally at least one D gene segment, and at least one J gene segment, wherein the variable region is capable of rearrangement for expressing a rearranged Ig variable domain; (b) an Ig locus intronic enhancer; and (c) a first Ig constant region encoding a first C domain; wherein the locus comprises a nucleotide sequence encoding a first self-associating mulimerization domain (SAM) (such as a self-associating tetramerization domain (TD)), the locus being operable to express a polypeptide comprising said rearranged V domain, SAM and C domain. A method of producing a nucleic acid for expressing a polypeptide comprising an EM1 and SAM, the method comprising (a) determining a nucleotide sequence of an EM1-encoding sequence comprised by a locus of a vertebrate of any one of Concepts 1-19, wherein EM1 is a V domain and the first nucleotide sequence comprises a rearranged V region encoding said V domain; and (b) producing a nucleic acid by operably connecting the sequence of step (a) with a nucleotide sequence encoding a SAM. The method of Concept 24, wherein the nucleic acid encodes a polypeptide comprising EM1, SAM and one or more antibody constant domains (eg, an antibody Fc region). The method of Concept 25, wherein the polypeptide comprises, in N- to C-terminal direction, (a) EM1, the constant domain(s) and SAM (optionally, EM1, Fc and SAM); or (b) EM1, SAM and the constant domain(s) (optionally, EM1, SAM and Fc). The method of Concept 25 or 26, wherein the polypeptide is an antibody heavy chain that comprises, in N- to C-terminal direction, EM1, Fc and SAM, wherein EM1 is optionally a VH. An expression vector or a host cell comprising a nucleic acid obtained by the method of any one of Concepts 24-27. The host cell of Concept 28, wherein the cell further comprises a nucleic acid encoding an antibody light chain, wherein the light chain comprises a second rearranged variable domain (eg, a VL), wherein the cell is capable of producing a multimer comprising two copies of a 4- chain antibody, wherein each antibody comprises a first copy of the heavy chain associated with a first copy of the light chain, a second copy of the heavy chain associated with a second copy of the light chain, and wherein the SAMs of the heavy chains associate together thereby forming the multimer. 30. The host cell of Concept 29, wherein in each antibody of the variable domains of the first heavy and light chains form a first antigen binding site; and the variable domains of the second heavy and light chains form a second antigen binding site. 31. A method of producing an antibody multimer, the method comprising expressing the multimer in a host cell according to Concept 29 or 30, and optionally isolating the multimer from the cell. 32. The method of Concept 31, further comprising formulating the isolated multimer to produce a pharmaceutical composition comprising the multimer and a pharmaceutically acceptable diluent, excipient or carrier. 33. A method of producing a polypeptide comprising EM1 and a SAM, or a method of producing a multimer of copies of a polypeptide comprising EM1 and a SAM, wherein EM1 is a protein domain that is capable of binding to a first antigen, the method comprising immunising a vertebrate according to any one of Concepts 1-19 with the first antigen, wherein the vertebrate expresses polypeptides or multimers comprising EM1 (that is capable of specifically binding to the antigen) and SAM, and obtaining a said polypeptide or multimer. 34. A method of obtaining a nucleotide sequence encoding an EM1, wherein EM1 is a protein domain that is capable of binding to a first antigen, the method comprising immunising a vertebrate according to Concept 3 or any one of Concepts 1-19 when dependent from Concept 3 with the first antigen, wherein the variable region of the locus undergoes rearrangement to produce a nucleotide sequence encoding EM1 (that is capable of specifically binding to the antigen), and obtaining said nucleotide sequence from a B-cell of the immunised vertebrate wherein the B-cell expresses a polypeptide comprising EM1. 35. The method of Concept 34 comprising producing an expression vector for expressing a polypeptide comprising EM1 and a SAM, wherein the method comprises operably linking the obtained nucleotide sequence to a nucleotide sequence encoding SAM. 36. A method for producing an expression vector for expressing a polypeptide comprising EM1 and a SAM, wherein the method comprises operably linking a nucleotide sequence that has been obtained by the method of Concept 34 to a nucleotide sequence encoding SAM. The vertebrate, cell(s), vector or cell of any preceding Concept, wherein each said polypeptide comprises an EM1 and a tetramerization domain (TD), wherein a first, a second, a third and a fourth copy of the polypeptide are capable of associating together by their TDs to form a tetramer comprising at least 4 copies of EM1. A multimer comprising a first, a second, a third and a fourth copy of the polypeptide recited in Concept 37, wherein EM1 is an Ig variable domain comprising an amino acid sequence that is encoded by the first nucleotide sequence of a vertebrate according to any one of Concepts 1-19 wherein the first nucleotide sequence comprises an Ig V region that has been rearranged in the vertebrate, wherein the vertebrate is a mouse and the amino acid sequence is encoded by a nucleotide sequence that comprises mouse pattern activation-induced cytidine deaminase (AID) and/or terminal deoxynucleotidyl transferase (TdT) mutation, optionally wherein the polypeptide comprises one or more human constant domains (such as an Fc region). A polyclonal polypeptide multimer population, wherein each multimer is a multimer of at least 3 or 4 copies of a respective polypeptide comprising an Ig V domain and a SAM, wherein SAMs comprised by each multimer are associated together, and wherein the population comprises at least 10 different types of said V domain of the polypeptides. The population of Concept 39, wherein each V domain is encoded by a respective nucleotide sequence that comprises rodent (eg, mouse or rat) pattern activation-induced cytidine deaminase (AID) and/or terminal deoxynucleotidyl transferase (TdT) mutation, and optionally which is obtainable from a vertebrate of any one of Concepts 1-19 that has been immunised by an antigen, wherein a plurality of said V domains are capable of binding to the antigen. A method for identifying or obtaining an antibody variable domain, a nucleotide sequence encoding an antibody variable domain, or an expression vector or host cell that is capable of expressing the domain, wherein the domain is capable of specifically binding to a target antigen, the method comprising (a) contacting the population of Concept 39 or 40 with said antigen; (b) binding multimers comprised by said population to said antigen; and (c) isolating or identifying one or more multimers that bind to the antigen, or isolating or identifying a said V domain of such a multimer; or identifying a nucleotide sequence encoding a V domain of such a multimer; and (d) optionally connecting the sequence to a nucleotide sequence encoding a SAM to produce a nucleic acid for expressing a polypeptide comprising the V domain and SAM; and optionally expressing and isolating said polypeptide or multimers thereof. 42. A non-human vertebrate (eg, mouse) blastocyst or pre-morula embryo implanted with an ES or iPS cell comprising a locus as recited in any one of Concepts 1-19, wherein the blastocyst or pre-morula embryo implanted with an ES or iPS cell is capable of developing into a vertebrate of any one of Concepts 1-19. 43. The vector of Concept 44 or 45, wherein the homology arms are homologous (or identical) to first and second sequences comprised by a constant region (eg, a gamma constant region) of said endogenous Ig locus, whereby the SAM can be inserted into the endogenous constant region. 44. A method of obtaining a humanised and affinity matured antigen-specific antibody heavy chain or a multimer thereof, the method comprising producing the heavy chain in vivo in a non-human vertebrate (eg, a mouse or a rat) comprising a functional activation induced cytidine deaminase (AID) by immunising the vertebrate with the antigen and obtaining somatic hypermutation and isotype switching in a B-cell of the vertebrate from an endogenous mu isotype to a human non-mu isotype, wherein an affinity matured antigen- specific antibody heavy chain is produced and expressed by the vertebrate, the non-mu constant domains of the heavy chain being human constant domains encoded by a non- mu constant region of the vertebrate and the heavy chain comprising a SAM encoded by the non-mu constant region, wherein copies of the heavy chain self-associated via SAMs to produce multimers and the method further comprises isolating at least one said multimer. 45. The method of Concept 44, wherein the SAM is fused to the 3’-most amino acid of a CH3 of the heavy chain. 46. The cell, vertebrate, multimer, population or method of any preceding Concept, wherein the polypeptide comprises an antibody heavy chain, the chain comprising in N- to C- terminal direction a rearranged V domain, an antibody Fc region and a SAM, wherein 4 copies of the polypeptide and 4 copies of a cognate light chain are associated together into a multimer, wherein the multimer comprises 2 copies of a 4-chain antibody wherein the heavy chains of the antibodies are associated together by 4 SAMs. 47. A nucleic acid (eg, a YAC) comprising a transgene for microinjection into a non-human vertebrate ES cell, the transgene comprising a locus as defined in any one of Concepts 1- 19. 48. A nucleic acid vector comprising a nucleic acid, wherein the nucleic acid comprises homology arms for recombination with the genome of a non-human vertebrate ES cell, wherein a nucleotide sequence encoding a SAM is between the homology arms, whereby said recombination is capable of inserting the nucleotide sequence into an endogenous Ig locus of the cell 3’ of an intronic enhancer thereof. 49. The vector of Concept 46, wherein the vector is a bacterial artificial chromosome (BAC). Features herein described in terms of a cell comprising a locus also apply mutatis mutandis to a vertebrate comprising the locus. Features herein described in terms of a vertebrate comprising a locus also apply mutatis mutandis to a cell comprising the locus. It will be understood that particular embodiments described herein are shown by way of illustration and not as limitations of the invention. The principal features of this invention can be employed in various embodiments without departing from the scope of the invention. Those skilled in the art will recognize, or be able to ascertain using no more than routine study, numerous equivalents to the specific procedures described herein. Such equivalents are considered to be within the scope of this invention and are covered by the claims. All publications and patent applications mentioned in the specification are indicative of the level of skill of those skilled in the art to which this invention pertains. All publications and patent applications and all US equivalent patent applications and patents are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. Reference is made to the publications mentioned herein and equivalent publications by the US Patent and Trademark Office (USPTO) or WIPO, the disclosures of which are incorporated herein by reference for providing disclosure that may be used in the present invention and/or to provide one or more features (eg, of a vector) that may be included in one or more claims herein. US11,453,726 discloses multimers, such as tetramers, of polypeptides that are multimerised using self-associating multimerisation domains (SAMs). In vitro expression of polypeptides comprising SAMs and assembly into useful multimers is disclosed. The entire disclosure of this application, including polypeptide and multimer formats, exemplary SAMs (eg, TDs) and effector moieties disclosed therein is incorporated herein by reference for use with the present invention. US20220162285 and US20190225710 also disclose polypeptides and multimers comprising SAMs; the entire disclosure of these applications are disclosed therein is incorporated herein by reference for use with the present invention. WO2004106375, US7919257, WO2009157771, EP2147594 and WO2014160179, US9796788, US2020214274, WO2011158009 and US10064398 discuss suitable techniques relevant to production of bispecific multimers and transgenic mice, as can be applied to the present invention; the entire disclosure of these applications are disclosed therein is incorporated herein by reference for use with the present invention. . The use of the word "a" or "an" when used in conjunction with the term "comprising" in the claims and/or the specification may mean "one," but it is also consistent with the meaning of "one or more," "at least one," and "one or more than one." The use of the term "or" in the claims is used to mean "and/or" unless explicitly indicated to refer to alternatives only or the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and "and/or." Throughout this application, the term "about" is used to indicate that a value includes the inherent variation of error for the device, the method being employed to determine the value, or the variation that exists among the study subjects. As used in this specification and claim(s), the words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "includes" and "include") or "containing" (and any form of containing, such as "contains" and "contain") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps. The term "or combinations thereof" or similar as used herein refers to all permutations and combinations of the listed items preceding the term. For example, "A, B, C, or combinations thereof is intended to include at least one of: A, B, C, AB, AC, BC, or ABC, and if order is important in a particular context, also BA, CA, CB, CBA, BCA, ACB, BAC, or CAB. Continuing with this example, expressly included are combinations that contain repeats of one or more item or term, such as BB, AAA, MB, BBC, AAABCCCC, CBBAAA, CABABB, and so forth. The skilled artisan will understand that typically there is no limit on the number of items or terms in any combination, unless otherwise apparent from the context. Any part of this disclosure may be read in combination with any other part of the disclosure, unless otherwise apparent from the context. All of the compositions and/or methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the invention. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims. The present invention is described in more detail in the following non-limiting Examples. EXAMPLES EXAMPLE 1: A transgenic mouse capable of expressing multivalent human antibodies Introduction Monoclonal antibodies (mAb) have become a powerful class of therapeutics owing to their exquisite selectivity, potency and pharmacokinetics profile. The race to develop mAbs as human therapeutics over the past four decades has led to the steady evolution of antibody discovery platforms. These include immunization of mice and the subsequent humanization of antibodies, use of phage display technology, generation of complex naive and synthetic antibody libraries through to engineered mice with fully humanized immunoglobulin (Ig) genes. The latter approach of using transgenic mice to generate human antibodies has proven to be the most successful given the streamlined discovery and development process. Humanized mice immunization with human target proteins are capable of generating highly potent and specific human antibodies that have undergone in vivo selection and maturation as well as ensuring proper Ig heavy chain and light chain pairing and thus this greatly reduces the risk of immunogenicity. Such naturally selected antibodies in vivo are ideal for developing therapeutic antibodies, as no additional protein engineering is required (Lee et al., 2014 & Murphy et al., 2014). Aims To date, humanized animal antibody platforms that have been engineered focused on humanizing the Ig genes to generate mice capable of producing fully human antibodies in their native formats. Our aim here is to engineer the next generation of humanized animal platforms that would allow production of multivalent antibody-based multimers that have undergone in vivo selection and maturation for stability, selectivity and potency. Engineering mice containing humanized Ig loci to include for example a sequence encoding a tetramerization domain (TD) such as p53 TD downstream of the Ig constant domains will give rise to a novel mouse platform capable of producing tetravalent human antibodies where copies of an antibody are associated together via TDs. Antibodies with the increased binding domain valency going from the standard bivalent IgG configuration to tetravalent configuration will greatly enhance the functional affinity of such antibodies. Mice challenged with a predeterimined antigen will give rise to an immune response producing a repertoire comprising antibodies with varying affinities and with broad epitope coverage. The standard antibody screening process using the resulting antigen-specific B cells will bias the discovery and selection towards high affinity antibodies against the most immunogenic epitopes whilst the low affinity antibodies against the low immunogenic rear epitopes would likely be lost. Production of multivalent antibodies such as tetravalent antibody multimers of the invention would greatly enhance the functional affinity (antigen binding avidity) of antibodies and this would increase the likelihood of identifying antibodies against rare epitopes that would have been otherwise difficult to identify due to their low affinity. Such multivalent human antibody-producing mice will allow for the discovery of potent antibody multimers in general compared to a standard humanized antibody-producing mouse as well as the discovery of antibodies with low affinity with broader epitope coverage. Generation of human multivalent antibody producing mice Humanization of the entire Ig variable (V) gene segment repertoire or only a subset of V gene segments in a mouse can be achieved using an engineering approach that is similar to the approach described previously to generate chimeric antibodies with human variable domains and mouse constant domains (Lee et al., 2014 & Murphy et al., 2014). In this way, as repertoire of rearranged human variable domains (EMs) in multimers of the invention can be obtained and screened against antigen using conventional techniques. Antigen-specific antibody multimers, their EM, or nucleotide sequence encoding EM can be isolated from such a mouse can be rapidly reformatted with human constant domain (comprising TD) in an expression vector to produce fully human antibody multimers. A conventional human antibody V domain-producing mouse can be readily transformed into a mouse of the invention that produces multimers comprising self-associated antibodies bearing human V domains by taking advantage of the class-switching process. When B cells encounter antigen, they become activated and start to proliferate and differentiate into effector cells and somatic DNA recombination takes place involving V(D)J recombination with antibody constant domains. IgM is the first class of antibody made containing μ heavy chains giving rise to pentameric antibodies. Upon subsequent antigen exposure B cells undergo isotype class switching with other heavy chains resulting in IgD, IgG, IgE, and/or IgA antibody production (Figure 1). To take advantage of the class switching process, a self- associating multimerisation domain (SAM) such as p53 TD or TTR TD can be fused to the C- terminus of the IgH constant regions such as Cγ1 region (Figure 2). This is done by introducing a nucleotide sequence encoding the SAM 3’ of the constant region exon encoding the Cγ1 CH3 domain. In such a mouse, the process of antigen recognition and B cell activation involving V(D)J recombination and production of IgM antibody would remain unchanged. However, it is only upon class switch recombination with Ig Cγ1 constant region fused to p53 TD domain that the mouse will be able to produce multivalent IgG1 antibody multimers (Figure 2). The introduction of a SAM (such as a TD domain) fused to IgH constant domains can be done through ES cell targeting using methodology described previously (Lee et al., 2014 & Murphy et al., 2014). A targeting vector can be used containing a sequence encoding SAM (eg, TD domain) and having a floxed positive/negative selection cassette such as puromycin- delta thymidine kinase (PuroDTK) with 5’ and 3’ homology arms that would direct integration of the targeting construct through homologous recombination preciously into the desired location in the constant region of a mouse IgH locus. Correctly integrated ES cell clones can be positively selected using puromycin selection and further verified through genotyping. Once positive clones are identified, the entire selection cassette can be excised from the genome by transiently expressing Cre-recombinase and negatively selecting with 1- (2-deoxy-2-fluoro-1-D-arabinofuranosyl)-5-iodouracil (FIAU) (Figure 3). Using a similar strategy, TD domains can be fused to other or all IgH constant regions (Figure 4). References A.J. Murphy, L.E. Macdonald, S. Stevens, M. Karow, A.T. Dore, K. Pobursky et al., Mice with megabase humanization of their immunoglobulin genes generate antibodies as efficiently as normal mice, Proc. Natl. Acad. Sci. U. S. A.111 (2014), pp.5153–5158. E.C. Lee, Q. Liang, H. Ali, L. Bayliss, A. Beasley, T. Bloomfield-Gerdes et al., Complete humanization of the mouse immunoglobulin loci enables efficient therapeutic antibody discovery, Nat. Biotechnol.32 (2014), pp.356–363. EXAMPLE 2: Construction of test targeting vectors for mouse ES cell targeting 2.1 pUC57_TEST-A_KI vector DNA synthesis TEST-A is a test polypeptide to be encoded by an engineered locus in a mouse that was constructed. A DNA fragment was constructed containing in 5’ to 3’ order a mouse intronic κ enhancer (miE_K), TEST-A encoding nucleotide sequence and a Cκ constant region. A 1-kb fragment between V_K and J_K region in the kappa locus, and a 1-kb fragment between mouse κ constant region (C_K) and mouse 3' κ enhancer (m3'E_K) were used as homology arms for targeting the TEST-A knock-in cassette. After successful targeting, kappa allele DNA including mouse C_K was replaced by the TEST-A knock-in. 2.2 pUC57_TEST-A_EFla_Puro_2A_EGFP_SV40pA targeting vector EFla_Puro_2A_EGFP_SV40pA vector was digested with NotI and Ascl. A 3.6 kb fragment was purified using QIAquick™ PCR Purification Ki (QIAGEN) by the method described in the attached instruction manual. pUC57_TEST-A_KI vector was digested with NotI and Ascl. A 7.9 kb fragment was purified QIAquick PCR Purification Kit (QIAGEN) by the method described in the attached instruction manual. The Notl-Ascl-digested EFla_Puro_2A_EGFP_SV40pA and pUC57_TEST-A_KI fragments were ligated using T4 ligase (New England Biolabs) according to the method described in the attached instruction manual. E.coli DH10B strain (ElectroMax™ DH10B (Invitrogen)) was transformed with the ligation solution. Respective plasmid DNAs were isolated from the obtained ampicillin resistant clones using QIAprep™ Spin Miniprep Kit (QIAGEN). The resulting respective ampicillin resistant transformants were confirmed to have the insertion by Sanger sequencing. 2.3 Preparation of plasmid DNA for mouse ES cell targeting For pUC57_TEST-A_EFla_Puro_2A_EGFP_SV40pA targeting vector, plasmid DNA was isolated from the sequencing verified clones using QIAprep Maxi plus Kit™ (QIAGEN) by the method described in the attached instruction manual. Respective plasmid DNAs were confirmed to have the insertion by Sanger sequencing. EXAMPLE 3. Generation of TEST-A knock-in mouse ES cell lines 3.1 Preparation of mouse ES cells Cells from two independent mouse embryonic stem cell (ES) lines were expanded on STO feeder plates for electroporation. ES cells were fed with fresh M15 media until they reached 80% to 85% confluence under microscope. The knock-in construct was introduced using electroporation. ES cells were cultured in M15 media supplemented with Puromycin (1 μg/mL). Seven days after electroporation, Puromycin resistant colonies were big enough for picking. 3.2 Microinjection of targeted mouse ES cell clones Targeted ES cells were injected to C57BL6 strain blastocysts. After injection, blastocysts were transferred to the uterus of C57BL6 strain Fl hybrid females mated with vasectomised males 3 days before injection. The chimaeric embryos were allowed to develop to term in the pseudo-pregnant recipients, which developed into pups comprising the light chain knock-in. EXAMPLE 4: Transgenic Mice Eight chimaera mice with TEST-A knock-in allele and a functional unrearranged human VH region in an IgH locus were immunized with human Target X. After immunization, sorting was performed on spleen B cells using a monoclonal antibody against TEST-A labelled with PE and human Target X labelled with AF647. B cells that were positive for both PE and AF647 were sorted as single cell into individual wells on a 96-well plate. Single cell NGS libraries were constructed using standard protocol and subjected to NGS sequencing. After subtraction of sequences from a PCR control included in our experiment, we found 346 clones, all of which comprised the expected TEST-A polypeptide sequence. Clustering the light chain sequences with that of the knock-in TEST-A allele demonstrated that 339 out of 346 clones were unmutated, with the other 7 clones having one or more mutations. Compared to sequences that are a perfect (ie, 100%) match for the inserted TEST-A sequence, ones that do not completely match have lower confidence scores (81.6% confidence score (for the average of the sequences assigned a 100% match) versus 31.7% confidence score (for the average of the 7 other sequences), 100% represents the highest confidence, 0% represents the lowest confidence). Thus, in an analysis we found 98% of sequences analysed comprised the identical input TEST-A sequence. After bioinformatics analysis, desirably the human heavy chain V domain sequence repertoire of the clones was found to be diverse. There were no dominant human V_H, D_H or J_H usage identified by the bioinformatics analysis. This matches the bulk NGS analysis of variable regions of heavy chains in spleen B cells from naive chimera mice (data not shown). Thus, it was shown that placement of TEST-A sequence could advantageously minimise somatic hypermutation during B-cell development in the mouse. This finding can usefully be borne in mind when deciding where to position the sequence encoding SAM in an Ig locus, with positioning of that sequence 3’ of the intronic enhancer in the locus appearing to be very beneficial to maintain the SAM sequence and thus function to self-multimerise to form multimers in the mouse of the invention. Similarly, placement of a sequence encoding EM2 or any other predetermined moiety 3’ of the intronic enhancer may be beneficial to minimise mutation of this desired moiety in the resultant multimers.

Table 1: Example Human Proteins Comprising a Tetramerization Domain

The amino acid and nucleotide sequences of each of these proteins and the TD thereof is incorportated herein by reference for use in the present invention and for potential inclusion in one or more claims herein.

Claims

CLAIMS: 1. A non-human vertebrate (eg, a mouse or a rat), wherein the genome of the vertebrate comprises a gene locus that encodes a polypeptide (optionally wherein the locus is an immunoglobulin (Ig) locus), wherein the locus comprises a first nucleotide sequence for producing a first effector moiety (EM1) and a second nucleotide sequence encoding a self-associating multimerisation domain (SAM), wherein the locus is capable of producing copies of a polypeptide comprising EM1 and SAM, wherein the SAM domains of the polypeptides associate together to produce a polypeptide multimer comprising a plurality of EM1 moieties.

2. The vertebrate of claim 1, wherein the first nucleotide sequence is an Ig locus variable region.

3. The vertebrate of claim 2, wherein the variable region is an unrearranged variable region that is capable of rearrangement to produce a nucleotide sequence encoding EM1.

4. The vertebrate of any preceding claim, wherein the locus comprises in 5’ to 3’ order an Ig intronic enhancer (eg, Eμ enhancer or Eiκ) and a second region (eg, a constant region), wherein the second region comprises the second nucleotide sequence encoding the SAM.

5. The vertebrate of any preceding claim, wherein the locus comprises a third nucleotide sequence for producing a second effector moiety (EM2), wherein the third sequence is 5’ of the first sequence; the third sequence is 3’ of the first sequence; or the third sequence is between the first and second sequences.

6. The vertebrate of claim 5, wherein EM1 is capable of binding to a first antigen or epitope; EM2 is capable of binding to a second antigen or epitope; and a) The first and second antigens are identical; b) The first and second epitopes are identical; c) The first and second antigens are different from each other; d) The first and second epitopes are different from each other.

7. The vertebrate of claim 5 or 6, wherein the first nucleotide sequence is an unrearranged Ig variable region and the third nucleotide sequence is a rearranged Ig variable region.

8. The vertebrate of any preceding claim, wherein the locus is an antibody locus or a T-cell receptor locus.

9. The vertebrate of any preceding claim, wherein each said effector moiety comprises a) a protein domain; or b) a peptide; optionally wherein each said effector moiety comprises an Ig variable domain (eg, an antibody VH or VL; or a TCR Vα, Vβ, Vγ or Vδ) or an epitope binding domain.

10. The vertebrate of claim 9, wherein each said effector moiety is an antibody single variable domain.

11. The vertebrate of any preceding claim, wherein SAM is a tetramerization domain (TD), optionally a p53, TTR, TPR, BCR or L27 TD.

12. The vertebrate of any preceding claim, wherein the second nucleotide sequence is comprised by an Ig constant region of the locus (optionally wherein the locus is an antibody locus and the second nucleotide sequence is comprised by a gamma constant region).

13. The vertebrate of any preceding claim (optionally according to claim 3), wherein the locus is an antibody heavy chain locus and comprises, in 5’ to 3’ direction, a) a first constant region of a first isotype (optionally a mu isotype); and b) a second constant region of a second isotype (optionally a non-mu isotype, such as a gamma isotype), wherein the second constant region comprises said second nucleotide sequence; wherein the locus is capable of isotype switching from the first isotype to the second isotype wherein the locus is capable of expressing polypeptides of the second isotype wherein each polypeptide comprises EM1 and a said SAM, optionally wherein the second constant region comprises a third nucleotide sequence that encodes a second effector moiety (EM2) wherein EM1 and EM2 are capable of binding to different antigens.

14. The vertebrate of claim 13, wherein a) the first constant region is devoid of a SAM; and/or b) the second constant region is devoid of a nucleotide sequence encoding a CH1.

15. The vertebrate of claim 13 or 14, wherein the second constant region comprises at its 3’ end the second nucleotide sequence encoding SAM, optionally wherein the second constant region comprises, in 5’ to 3’ order, an exon encoding a CH3 and the second nucleotide sequence encoding SAM.

16. The vertebrate of claim 13, wherein the locus is capable of producing a polypeptide selected from a polypeptide comprising, in N- to C-terminal direction, a) EM1 and SAM; b) SAM and EM1; c) EM1, EM2 and SAM; d) SAM, EM1 and EM2 e) EM1, SAM and EM2; f) EM1, EM2, EM3 and SAM; g) SAM, EM1, EM2 and EM3; h) EM1,EM2 SAM and EM3; i) EM1 SAM, EM2 and EM3; j) EM1, EM2, EM3, EM4 and SAM; k) SAM, EM1, EM2, EM3 and EM4; l) EM1, EM2, EM3 SAM and EM4; m) EM1 SAM, EM2, EM3 and EM4; n) EM1, EM2 SAM, EM3 and EM4; o) EM1, EM2, EM3, EM4, EM5 and SAM; p) SAM, EM1, EM2, EM3, EM4 and EM5; q) EM1, EM2, EM3 SAM, EM4 and EM5; r) EM1, EM2 SAM, EM3, EM4 and EM5; s) EM1, EM2, EM3 SAM, EM4 and EM5; t) EM1, EM2, SAM, EM3, EM4 and EM5; and u) EM1, EM2, EM3, SAM, EM4, EM5 and EM6; Wherein each EM is an effector moiety (optionally an Ig variable domain or peptide); and optionally wherein EM1 is capable of binding to a first antigen or epitope; EM2 is capable of binding to a second antigen or epitope; and a) The first and second antigens are identical; b) The first and second epitopes are identical; c) The first and second antigens are different from each other; d) The first and second epitopes are different from each other.

17. A non-human vertebrate or a non-human vertebrate cell that comprises a rearranged Ig variable region (eg, an VH or VL region) encoding a V domain, wherein the variable region is ectopically positioned in the genome of the vertebrate or cell, wherein the vertebrate or cell expresses polypeptides each comprising a SAM and a said V domain, wherein the V domains of the polypeptides comprise most commonly a CDR3 length of 11 to 16 (eg, 15) amino acids.

18. A non-human vertebrate (eg, a mouse) according to any one of claims 1-17 or a non- human vertebrate cell (eg, a mouse cell) whose genome comprises a locus for expression of antibody heavy chains, the locus comprising a) an unrearranged human variable region comprising human variable region gene segments for expression of a repertoire of human variable domains (EM1 moieties); b) an endogenous mu constant region for expression of IgM antibody heavy chains comprising endogenous mu heavy chain constant domains and human variable domains; and c) (i) a humanised non-mu constant region downstream of the mu constant region for expression of non-mu antibody heavy chains comprising human non-mu constant domains and human variable domains, wherein the non-mu constant region comprises a nucleotide sequence encoding a SAM; or (ii) a non-human vertebrate (eg, mouse) non-mu constant region downstream of the mu constant region for expression of non-mu antibody heavy chains comprising non-human vertebrate (eg, mouse) non-mu constant domains and human variable domains, wherein the non-mu constant region comprises a nucleotide sequence encoding a SAM; wherein the unrearranged variable region is provided as a targeted insertion of the human variable region gene segments upstream of the endogenous mu constant region in an endogenous IgH locus such that the variable region gene segments are able to recombine for expression and selection in the context of an endogenous mu constant region.

19. A non-human vertebrate (eg, a mouse or a rat) according to any one of claims 1-17 or a non-human vertebrate cell whose genome comprises an antibody heavy chain locus comprising (in 5′ to 3′ direction) a variable region, a first switch, an endogenous mu constant region, a second switch and a human non-mu (eg, gamma) constant region, wherein the non-mu constant region comprises a nucleotide sequence encoding a SAM; wherein the heavy chain locus of each cell is capable of undergoing switching from IgM to the non-mu (eg, IgG) isotype for the production of non-mu heavy chains comprising a rearranged V domain and a SAM.

20. A cell or a plurality of cells, wherein each cell is a) an isolated lymphocytic cell (optionally a B- or T-cell) obtainable from a vertebrate of any preceding claim, each comprises a locus as defined in any preceding claim for producing polypeptides comprising EM1 and a SAM.; or b) an embryonic stem cell or pluripotent stem cell that can develop into a vertebrate according to any preceding claim.

21. The cell(s) of claim 20(a), wherein the germline of the vertebrate genome comprises a locus according to claim 13 when dependent from claim 3 and each said cell comprises an isotype switched locus obtainable by switching the locus recited in claim 13 to the second isotype, wherein the switched locus comprises a rearranged variable region encoding EM1 and produced by rearrangement of the variable region recited in claim 3, wherein each said cell produces a respective polypeptide comprising an EM1 and a SAM.

22. The cell(s) of claim 21, wherein each said cell is capable of secreting a multimer of its respective polypeptide, wherein copies of the polypeptide are associated by SAMs.

23. A lymphocyte cell obtainable from the vertebrate of any one of claims 1-19 and comprising a locus for expressing a polypeptide, wherein the locus comprises (in 5' to 3' direction) (a) an unrearranged Ig variable region comprising at least one V gene segment, optionally at least one D gene segment, and at least one J gene segment, wherein the variable region is capable of rearrangement for expressing a rearranged Ig variable domain; (b) an Ig locus intronic enhancer; and (c) a first Ig constant region encoding a first C domain; wherein the locus comprises a nucleotide sequence encoding a first self-associating mulimerization domain (SAM) (such as a self-associating tetramerization domain (TD)), the locus being operable to express a polypeptide comprising said rearranged V domain, SAM and C domain.

24. A method of producing a nucleic acid for expressing a polypeptide comprising an EM1 and SAM, the method comprising (a) determining a nucleotide sequence of an EM1-encoding sequence comprised by a locus of a vertebrate of any one of claims 1-19, wherein EM1 is a V domain and the first nucleotide sequence comprises a rearranged V region encoding said V domain; and (b) producing a nucleic acid by operably connecting the sequence of step (a) with a nucleotide sequence encoding a SAM.

25. The method of claim 24, wherein the nucleic acid encodes a polypeptide comprising EM1, SAM and one or more antibody constant domains (eg, an antibody Fc region).

26. The method of claim 25, wherein the polypeptide comprises, in N- to C-terminal direction, (a) EM1, the constant domain(s) and SAM (optionally, EM1, Fc and SAM); or (b) EM1, SAM and the constant domain(s) (optionally, EM1, SAM and Fc).

27. The method of claim 25 or 26, wherein the polypeptide is an antibody heavy chain that comprises, in N- to C-terminal direction, EM1, Fc and SAM, wherein EM1 is optionally a VH.

28. An expression vector or a host cell comprising a nucleic acid obtained by the method of any one of claims 24-27.

29. The host cell of claim 28, wherein the cell further comprises a nucleic acid encoding an antibody light chain, wherein the light chain comprises a second rearranged variable domain (eg, a VL), wherein the cell is capable of producing a multimer comprising two copies of a 4- chain antibody, wherein each antibody comprises a first copy of the heavy chain associated with a first copy of the light chain, a second copy of the heavy chain associated with a second copy of the light chain, and wherein the SAMs of the heavy chains associate together thereby forming the multimer.

30. The host cell of claim 29, wherein in each antibody of the variable domains of the first heavy and light chains form a first antigen binding site; and the variable domains of the second heavy and light chains form a second antigen binding site.

31. A method of producing an antibody multimer, the method comprising expressing the multimer in a host cell according to claim 29 or 30, and optionally isolating the multimer from the cell.

32. The method of claim 31, further comprising formulating the isolated multimer to produce a pharmaceutical composition comprising the multimer and a pharmaceutically acceptable diluent, excipient or carrier.

33. A method of producing a polypeptide comprising EM1 and a SAM, or a method of producing a multimer of copies of a polypeptide comprising EM1 and a SAM, wherein EM1 is a protein domain that is capable of binding to a first antigen, the method comprising immunising a vertebrate according to any one of claims 1-19 with the first antigen, wherein the vertebrate expresses polypeptides or multimers comprising EM1 (that is capable of specifically binding to the antigen) and SAM, and obtaining a said polypeptide or multimer.

34. A method of obtaining a nucleotide sequence encoding an EM1, wherein EM1 is a protein domain that is capable of binding to a first antigen, the method comprising immunising a vertebrate according to claim 3 or any one of claims 1-19 when dependent from claim 3 with the first antigen, wherein the variable region of the locus undergoes rearrangement to produce a nucleotide sequence encoding EM1 (that is capable of specifically binding to the antigen), and obtaining said nucleotide sequence from a B-cell of the immunised vertebrate wherein the B-cell expresses a polypeptide comprising EM1.

35. The method of claim 34 comprising producing an expression vector for expressing a polypeptide comprising EM1 and a SAM, wherein the method comprises operably linking the obtained nucleotide sequence to a nucleotide sequence encoding SAM.

36. A method for producing an expression vector for expressing a polypeptide comprising EM1 and a SAM, wherein the method comprises operably linking a nucleotide sequence that has been obtained by the method of claim 34 to a nucleotide sequence encoding SAM.

37. The vertebrate, cell(s), vector or cell of any preceding claim, wherein each said polypeptide comprises an EM1 and a tetramerization domain (TD), wherein a first, a second, a third and a fourth copy of the polypeptide are capable of associating together by their TDs to form a tetramer comprising at least 4 copies of EM1.

38. A multimer comprising a first, a second, a third and a fourth copy of the polypeptide recited in claim 37, wherein EM1 is an Ig variable domain comprising an amino acid sequence that is encoded by the first nucleotide sequence of a vertebrate according to any one of claims 1-19 wherein the first nucleotide sequence comprises an Ig V region that has been rearranged in the vertebrate, wherein the vertebrate is a mouse and the amino acid sequence is encoded by a nucleotide sequence that comprises mouse pattern activation-induced cytidine deaminase (AID) and/or terminal deoxynucleotidyl transferase (TdT) mutation, optionally wherein the polypeptide comprises one or more human constant domains (such as an Fc region).

39. A polyclonal polypeptide multimer population, wherein each multimer is a multimer of at least 3 or 4 copies of a respective polypeptide comprising an Ig V domain and a SAM, wherein SAMs comprised by each multimer are associated together, and wherein the population comprises at least 10 different types of said V domain of the polypeptides.

40. The population of claim 39, wherein each V domain is encoded by a respective nucleotide sequence that comprises rodent (eg, mouse or rat) pattern activation-induced cytidine deaminase (AID) and/or terminal deoxynucleotidyl transferase (TdT) mutation, and optionally which is obtainable from a vertebrate of any one of claims 1-19 that has been immunised by an antigen, wherein a plurality of said V domains are capable of binding to the antigen.

41. A method for identifying or obtaining an antibody variable domain, a nucleotide sequence encoding an antibody variable domain, or an expression vector or host cell that is capable of expressing the domain, wherein the domain is capable of specifically binding to a target antigen, the method comprising (a) contacting the population of claim 39 or 40 with said antigen; (b) binding multimers comprised by said population to said antigen; and (c) isolating or identifying one or more multimers that bind to the antigen, or isolating or identifying a said V domain of such a multimer; or identifying a nucleotide sequence encoding a V domain of such a multimer; and (d) optionally connecting the sequence to a nucleotide sequence encoding a SAM to produce a nucleic acid for expressing a polypeptide comprising the V domain and SAM; and optionally expressing and isolating said polypeptide or multimers thereof.

42. A non-human vertebrate (eg, mouse) blastocyst or pre-morula embryo implanted with an ES or iPS cell comprising a locus as recited in any one of claims 1-19, wherein the blastocyst or pre-morula embryo implanted with an ES or iPS cell is capable of developing into a vertebrate of any one of claims 1-19.

43. A method of obtaining a humanised and affinity matured antigen-specific antibody heavy chain or a multimer thereof, the method comprising producing the heavy chain in vivo in a non-human vertebrate (eg, a mouse or a rat) comprising a functional activation induced cytidine deaminase (AID) by immunising the vertebrate with the antigen and obtaining somatic hypermutation and isotype switching in a B-cell of the vertebrate from an endogenous mu isotype to a human non-mu isotype, wherein an affinity matured antigen- specific antibody heavy chain is produced and expressed by the vertebrate, the non-mu constant domains of the heavy chain being human constant domains encoded by a non- mu constant region of the vertebrate and the heavy chain comprising a SAM encoded by the non-mu constant region, wherein copies of the heavy chain self-associated via SAMs to produce multimers and the method further comprises isolating at least one said multimer.

44. The method of claim 43, wherein the SAM is fused to the 3’-most amino acid of a CH3 of the heavy chain.

45. The cell, vertebrate, multimer, population or method of any preceding claim, wherein the polypeptide comprises an antibody heavy chain, the chain comprising in N- to C-terminal direction a rearranged V domain, an antibody Fc region and a SAM, wherein 4 copies of the polypeptide and 4 copies of a cognate light chain are associated together into a multimer, wherein the multimer comprises 2 copies of a 4-chain antibody wherein the heavy chains of the antibodies are associated together by 4 SAMs.

46. A nucleic acid (eg, a YAC) comprising a transgene for microinjection into a non-human vertebrate ES cell, the transgene comprising a locus as defined in any one of claims 1-19.

47. A nucleic acid vector comprising a nucleic acid, wherein the nucleic acid comprises homology arms for recombination with the genome of a non-human vertebrate ES cell, wherein a nucleotide sequence encoding a SAM is between the homology arms, whereby said recombination is capable of inserting the nucleotide sequence into an endogenous Ig locus of the cell 3’ of an intronic enhancer thereof.

48. The vector of claim 47, wherein the vector is a bacterial artificial chromosome (BAC).