The present invention relates inter alia to a rodent or rodent cell having a genome comprising: i) one or more companion animal IGH V region genes, one or more companion animal D region genes and one or more companion animal J region genes; and (ii) optionally one or more companion animal IGL kappa V region genes and one or more companion animal IGL kappa J region genes; and/or one or more companion animal IGL lambda V region genes and one or more companion animal IGL lambda J region genes, wherein the rodent or rodent cell is capable of expressing the companion animal variable region gene(s) to form an antibody chain and wherein the companion animal species is not a rodent. The companion animal genes are preferably canine. Also described are methods for producing such cells, methods for producing antibody chains using the cells, and methods of treating diseases of companion animals by administration of the antibodies.
