The following sequences were inserted between Igkj segments and the Igk-Eik super-enhancer: murine Igh core switch region mu, human IGK-EIK super-enhancer, human IGK constant region, Igkv 4-63 promoter sequence and 1st exon with a STOP sequence, murine Igh core switch region gamma3, and a loxP site flanked neomycin resistance gene cassette. Subsequent cre-mediated recombination removed the neo cassette. (J:224373)