In particular, NARP is most frequently caused by heteroplasmic point mutations in MT-ATP6, an mtDNA gene encoding subunit 6 of mitochondrial H (+)-ATPase (complex V); 70–90% of mutation loads usually manifest as NARP syndrome; meanwhile, higher levels of mutants may lead to MILS [54,55]. The gene discussed is MT-ATP6; the disease is maternally-inherited Leigh syndrome.