The invention provides A mouse whose genome comprises a transgene comprising a polynucleotide sequence comprising DNA sequences corresponding to DNA sequences from a mouse immunoglobulin locus except for polynucleotide sequences encoding one or more human immunoglobulin light chain variable (V) polypeptide sequences and one or more human immunoglobulin light chain joining (J) polypeptide sequences, wherein the sequences encoding the one or more human light chain V polypeptide sequences and the one or more human light chain J polypeptide sequences replace one or more mouse immunoglobulin light chain V coding sequences and one or more mouse immunoglobulin light chain J coding sequences, respectively.