(A) Diagram showing Nkx2-5 conditional mouse construct. (B) Southern blot analysis of targeted embryonic stem cells showing recombination using 5′ and 3′ probes. Nc, NcoI; P, PvuII; X, XbaI; N, NotI; Xh, XhoI; M, MluI; Bg, BglII; E, EchoRI; 5′P, 5′Southern probe; 3′P, 3′ Southern probe; LoxP, Cre-recombinase cleavage sites; PGK-Neo, phosphoglycerate kinase promoter, neomycin resistance gene.