These data show that a primary CoQ10 defect can be associated with MRC enzymatic deficiency because patient P01, who carried a deleterious homozygous mutation (c.437G > A; p.Ser146Asn) in the CoQ2 gene, also presented a complex IV deficiency in muscle. This evidence concerns the gene COQ2 and hyperinsulinemic hypoglycemia, familial, 4.