Although it is thought that the induction of FGF21 expression and release by skeletal muscle under conditions, such as pathogenic mitochondrial dysfunction, is responsible, in whole or in part, for the high systemic levels of FGF21 in patients with mitochondrial diseases, whether cardiac production of FGF21 results in altered systemic levels in physiological or pathogenic conditions studied to date remains unknown. The gene discussed is FGF21; the disease is inborn mitochondrial metabolism disorder.