Phenotypic variations in disease severity depend mainly on the fraction of mutated mtDNA percent (i.e., the level of heteroplasmy), as observed in Maternally inherited Leigh syndrome (MILS) and neuropathy, ataxia and retinis pigmentosa (NARP) syndrome, caused by subunit a (MT-ATP6) mutations. This evidence concerns the gene MT-ATP6 and maternally-inherited Leigh syndrome.