A transgenic mouse is provided comprising in its genome an endogenous λ light chain immunoglobulin locus comprising a deletion of the endogenous mouse Vλ1-Jλ3-Cλ3-Jλ1-Cλ1 gene cluster and a deletion of a region spanning from Vλ2 to Jλ2 of the endogenous mouse Vλ2-Vλ3-Jλ2-Cλ2-Jλ4-Cλ4 gene cluster. Mouse cells and mouse embryos are also provided.