PGC1β−/− appear normal, while DKO resulted in neonatal lethality with heart failure (HF). TEM of the E16.5, E17.5, PD0.5 hearts show lack of increase of heart size showed ↑ of mitochondria in WT, single KO, but not DKO hearts; ↓ mitochondrial volume density and normal myofibril volume density at PD0.5 The gene discussed is PPARGC1B; the disease is heart failure.