Consistent with these results, there was a 20% reduction in mitochondrial to nuclear DNA ratio, as determined by real-time PCR (Amthor et al., 2007), in Rb???/??? versus control cultures at DM-2 (P < 0.001; Fig. 2 F) and a 50% reduction in E16.5 limb muscle (P = 0.009; Fig. 2 G).