Strategy for the construction of an engineered κ light chain locus. Structure of the targeted locus (not to scale). (Top) wt allele showing an unrearranged Igκ locus, the extent of the Jκ deletion and the localization of the 3′ probe (1.6-kb Afl2 fragment). (Middle) Structure of mutated locus (κ-Neo mutation) after recombination with the targeting vector (in which a Neo^r cassette flanked by 2 LoxP sites followed with a pVH promoter and with the complete coding sequence of the VκJκ CHEB rearranged exon was used to replace the Jκ cluster); dashed lines indicate the positions of homologous sequences used as 5′ and 3′ arms of the targeting construct. (Bottom) The resulting locus after Cre-mediated deletion of either only the neo^r cassette (κ-CHEB mutation) or both the CHEB VκJκ exon and the Neo^r cassette (κ-del mutation). B indicates BamHI; Bs, BsmI; and S, SacI. Only the relevant restriction sites are shown. Short arrows represent the position of primers used for PCR as described in “Materials and methods.” 1 indicates 5′ Jκ; 2,3′ Jκ; and 3, Neo3.