Genotype of offspring from interbreeding mice heterozygous for the targeted TGF-β1 allele. Tail DNAs from parents (116 and 119) and offspring (159–167) were digested with StuI, fractionated by electrophoresis through 0.8% agarose, transferred to nylon membranes, and hybridized with a TGF-β1 probe (probe B; Fig. 1). The upper band (6.7 kb) represents the wild-type allele and the lower band (2.5 kb) represents the targeted allele. +/+, Homozygous wild-type; +/−, heterozygote; −/−, homozygous targeted.