Map of the replacement targeting vector and Southern blot of mouse tail DNA. (A) Wild-type genomic locus, genomic clone B122, targeting vector pBC1/Neo-TK, and modified genomic locus lacking the BC1 RNA gene. Neo, neomycin phosphotransferase gene; HSV-TK, TK gene under control of the simian virus 40 promoter. The bars indicate probes for Southern blot hybridization: HR, homologous recombination probe; Neo, probe specific for the neomycin phosphotransferase gene. B, BglII; E, EcoRI; V, EcoRV; H, HindIII; P, PstI. (B) Southern blot analysis of PstI-digested genomic DNAs from nine different pups after crossing of two heterozygous parents. Mice 4 and 9 possess both wild-type alleles (12 kb); mice 1, 2, 6, and 8 are heterozygous; and mice 3, 5, and 7 have both gene deletion alleles (4.8 kb).