Mfn2-AA mice have smaller, abnormally shaped mitochondria in the heart starting at 3 weeks of age, associated with ↓ succinate dehydrogenase (SDHB) complex II subunit protein compared to wild-type (WT) Mfn2 transgenics, at 5 weeks of age, the Mfn2-AA mutant mice develop DCM. The gene discussed is SDHB; the disease is familial dilated cardiomyopathy.