Fig. 1. Generation of BAenh^–/– mice. (A) Targeting strategy. A 754 bp XhoI-BamHI fragment containing a 208 bp branchial arch-specific enhancer was replaced with a neo^r cassette flanked by loxP sites (triangles), introducing an XbaI site at the 5' end of the cassette. The +neoBAenh mutant allele contains a neo^r cassette. Cre-mediated recombination of this allele generated the neoBAenh mutant allele. tk, thymidine kinase; S, SacI; E, EcoRI; Xh, XhoI; B, BamHI; N, NotI; Nd, NdeI; P, PstI; Xb, XbaI. (B) Southern blot analysis of tail DNA digested with SacI and hybridized with a 5' probe (upper panel), or digested with NdeI and XbaI and hybridized with a 3' probe (lower panel). The genotype is listed on the top of each lane. (C) PCR genotype of tail DNA. Each panel shows a PCR reaction amplifying a 500 bp neomycin gene (neo), a 300 bp fragment containing the branchial arch enhancer of dHAND (BAen), a 300 bp Cre-recombinase fragment (cre) and a 302 bp control sequence (cont).