Deletion of the Iμ-Cμ intron. (A) Structure of the targeted locus (not to scale). (Top) Wild-type allele showing an unrearranged IgH locus, the extent of the deletion, and the location of the 3′ probe (0.6-kb XhoI-XbaI fragment). (Middle) Structure of the targeting vector in which a neo^r cassette flanked by 2 loxP sites was used to replace the Iμ-Cμ intron. The proximal XhoI site was picked from the phage multiple cloning site; the endogenous StuI and SpeI sites were replaced by NotI and ClaI sites, respectively. (Bottom) The resulting locus after Cre-mediated deletion of neo^r cassette. C indicates ClaI; H, HindIII; N, NotI; R, EcoRI; S, SpeI; St, StuI; X, XbaI; and Xh, XhoI. Only the relevant restriction sites are shown. (B) Southern blot analysis of wild-type and mutant mice. High–molecular weight DNA was extracted from the indicated mice tails and digested with EcoRI. The 3′ probe distinguishes the germ line band (∼11 kb) from the neo^r recombinant band (∼7 kb) and the Δ recombinant band (∼6 kb) and enables detection of a polymorphism between 129 and C57Bl/6 strains. (C) Nucleotide sequence of the Iμ-Cμ intron after removal of the neo^r cassette. The distal part of Iμ exon and the proximal part of the Cμ1 exon are shown in bold capital letters. The remaining germ line intronic sequences are shown in normal capital letters; the canonical donor and acceptor splice sites are underlined. The exogenous sequences brought by the targeting vector are shown in lowercase letters with the loxP sequence in bold.