A method for producing a rodent able to produce a repertoire of chimaeric antibodies or heavy antibody chains, the method comprising: inserting into a rodent cell genome; (a) a plurality of human IgH V regions, one or more human D regions and one or more human J regions upstream of a constant region of the host rodent; and (b) optionally one or more human Ig light chain kappa V regions and one or more human Ig light chain kappa J regions upstream of the host rodent mammal kappa constant region and/or one or more human Ig light chain lambda V regions and one or more human Ig light chain lambda J regions upstream of the host rodent mammal lambda constant region; respectively, the insertion being such that the rodent is able to produce a repertoire of chimaeric antibodies or heavy antibody chains having a rodent constant region and a human variable region, wherein steps (a) and (b), when both present, can be carried out in either order, wherein, one or more insertion events utilises site specific recombination, wherein the insertion process commences at a site where an initiation cassette has been inserted into the genome of the rodent cell, such as an ES cell, and wherein the insertion of a first DNA fragment into the initiation cassette is followed by insertion of a second DNA fragment into a portion of the first DNA fragment, and wherein the insertion of the human DNA is made between the rodent constant region and the last, 3', rodent J region. inserting into a rodent cell genome; (a) a plurality of human IgH V regions, one or more human D regions and one or more human J regions upstream of a constant region of the host rodent; and (b) optionally one or more human Ig light chain kappa V regions and one or more human Ig light chain kappa J regions upstream of the host rodent mammal kappa constant region and/or one or more human Ig light chain lambda V regions and one or more human Ig light chain lambda J regions upstream of the host rodent mammal lambda constant region; respectively, (a) a plurality of human IgH V regions, one or more human D regions and one or more human J regions upstream of a constant region of the host rodent; and (b) optionally one or more human Ig light chain kappa V regions and one or more human Ig light chain kappa J regions upstream of the host rodent mammal kappa constant region and/or one or more human Ig light chain lambda V regions and one or more human Ig light chain lambda J regions upstream of the host rodent mammal lambda constant region; respectively, the insertion being such that the rodent is able to produce a repertoire of chimaeric antibodies or heavy antibody chains having a rodent constant region and a human variable region, wherein steps (a) and (b), when both present, can be carried out in either order, wherein, one or more insertion events utilises site specific recombination, wherein the insertion process commences at a site where an initiation cassette has been inserted into the genome of the rodent cell, such as an ES cell, and wherein the insertion of a first DNA fragment into the initiation cassette is followed by insertion of a second DNA fragment into a portion of the first DNA fragment, and wherein the insertion of the human DNA is made between the rodent constant region and the last, 3', rodent J region.