Compound heterozygous mutations in the SDHD gene were first reported in a girl with encephalomyopathy and biochemical evidence of isolated mitochondrial complex II deficiency who died at 10 years, and homozygous SDHD mutation in an infant with fatal cardiomyopathy and mitochondrial complex II deficiency [125,126]. Here, SDHD is linked to hyperinsulinemic hypoglycemia, familial, 4.