Fig. 4. Deletion of Myog and attenuation of myogenin transcripts and protein in Myg^flox/flox mice. (A) Quantitative PCR of tail DNA to determine the extent of deletion of floxed Myog sequences in Myog^flox/flox and Myog^flox/flox mice at P10. The ratio of Myog to Myf5 genomic DNA from one Myog^flox/flox mouse was arbitrarily set to 1 and all other ratios from both Myog^flox/flox and Myog^flox/flox genotypes were normalized to that value. (B) Southern genome hybridization with EcoRI-digested DNA extracted from the hindlimb skeletal muscle of 10-week-old mice from a Myog^flox/flox;+/+ x Myog^flox/flox;CAGGCre-ER^TM/+ cross and probed with Myog cDNA. The 6 kb and 3 kb bands represent the Myog^flox and Myog^flox alleles, respectively, as depicted in Fig. 1. The skeletal DNA in lanes 1 and 3 contained very little of the Myog^flox allele and therefore represents a Myog^flox/flox genotype, while the skeletal DNA in lanes 2, 4, 5 and 6 contained no Myog^flox allele and therefore represents a Myog^flox/flox genotype. (C) Quantitative RT-PCR to determine the levels of Myog transcript expression in hindlimb skeletal muscle from mice at 3 days (right panel) and 2 weeks (left panel) of age. The genotypes are shown below the histograms. The ratio of Myog to ribosomal protein L7 RNA for one RNA sample with a Myog^flox/flox genotype was arbitrarily set to 1, and all other ratios from both Myog^flox/flox and Myog^flox/flox genotypes were normalized to that value. The median value for each genotype is shown as a black dot. Asterisks indicate significant differences between the two genotypes. The arrow bars indicate one s.d. from the median. (D) Hindlimb sections from Myog^flox/flox (right panel) and Myog^flox/flox (left panel) mice at 3 days of age immunostained with the monoclonal F5D anti-myogenin antibody. Arrows indicate positively stained cells. Scale bar: 100 µm.