Disclosed is a mouse whose genome comprises: (a) a plurality of human IgH V regions, one or more human D regions and one or more human J regions upstream of a mouse constant region (b) one or more human Ig light chain kappa V regions and one or more human Ig light chain kappa J regions upstream of a mouse kappa constant region and optionally one or more human Ig light chain lambda V regions and one or more human Ig light chain lambda J regions upstream of a mouse lambda constant region wherein the mouse is able to produce a repertoire of chimaeric antibodies, or chimaeric heavy chains and chimaeric light chains, having a mouse constant region and a human variable region wherein the mouse genome comprises a chimaeric antibody chain locus, the locus encoding a chimaeric heavy chain and wherein the locus comprises a mouse constant region nucleotide sequence and a rearranged VDJ nucleotide sequence produced by in vivo rearrangement of a human V region, a human D region and a human J region - the V region being a V1-3 segment and - the mouse expressing more V1-3 JH4 or V1-3 JH6 antibodies than any of, individually, V1-3 JH1, V1-3 JH2, V1-3 JH3 or V1-3 JH5 antibodies.
