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, and murine Igh core switch region gamma3. The Rr152 (c3'RR) Igh enhancer cluster and a loxP site flanked neomycin resistance gene cassette were inserted downstream of the Igkc segment. Subsequent cre-mediated recombination removed the neo cassette. (J:302182)