Herein is reported a transgenic vector comprising a humanized light chain locus, wherein said humanized light chain locus comprises (a) a V gene segment derived from human light chain V segment IGKV1-39-01, (b) 3' proximal to said light chain gene segment a promoter, and (c) 5' proximal to said light chain gene segment at least a fragment of the human IGKJ4 J-element.