Mitochondrial DNA (mtDNA) mutations in cytochrome b (MT-CYB) gene constitute a major cause of complex III deficiency and underlie a wide range of neuromuscular disorders (Gil Borlado et al., 2010), with exercise intolerance as a major symptom (Andreu et al., 1999). The gene discussed is MT-CYB; the disease is hyperinsulinemic hypoglycemia, familial, 4.