FIS1 and cardiomyopathy: 30% levels of PGC1β mRNA at birth in the heart. Mice start to die at 5 weeks of age with 14% survival by 20 weeks. Progressive cardiomyopathy starting at 1 week of age. Mitochondria appear normal postnatal day 1, fragmentation and elongation abnormality at 1 week, and progressively worsen and also observed ↓ mtDNA, mito protein, and mito function. ↓ Mfn1, Mfn2, Opa1, Fis1 and Drp1, Mitophagosomes were not observed.