A transgenic mouse is provided comprising in its genome an endogenous » light chain immunoglobulin locus comprising a deletion of the endogenous mouse V»1-J»3-C»3-J»1-C»1 gene cluster and a deletion of a region spanning from V»2 to J»2 of the endogenous mouse V»2-V»3-J»2-C»2-J»4-C»4 gene cluster. Mouse cells and mouse embryos are also provided.