A rare, genetic, mitochondrial oxidative phosphorylation disorder that may present with a wide range of symptoms (including muscular hypotonia, hypertrophic cardiomyopathy, psychomotor delay, encephalopathy, peripheral neuropathy, lactic acidosis, 3-methylglutaconic aciduria) and clinical syndromes (including NARP and MILS). The disease is mitochondrial proton-transporting ATP synthase complex deficiency (Monarch Disease Ontology entry MONDO_0014471). Also known as: mitochondrial complex V (ATP synthase) deficiency.