COQ9 and hypertrophic cardiomyopathy: The patient harbored a homozygous nonsense mutation in COQ9, resulting in a truncated COQ9 polypeptide (Arg244Ter), presented with neonatal lactic acidosis, and later developed multisystem disease including intractable seizures, global developmental delay, hypertrophic cardiomyopathy, and renal tubular dysfunction.