One patient was found to have Leigh syndrome due to a mutation in MT‐ATP6, two affected siblings were discovered to be compound heterozygous for mutations in the NDUFV1 gene, which causes mitochondrial complex I deficiency, and one patient was found to have coenzyme Q10 deficiency due to compound heterozygous mutations in COQ2. Here, MT-ATP6 is linked to hyperinsulinemic hypoglycemia, familial, 4.