Variable heteroplasmy levels of the same mtDNA variant can even lead to phenotypic expression corresponding to different clinical syndromes—for example, the same mutation in MT-ATP6 gene with a mutation load of 60–90% causes the less severe NARP, while a load of more than 90% causes the more clinically severe maternally inherited Leigh syndrome (MILS) [11]. The gene discussed is MT-ATP6; the disease is maternally-inherited Leigh syndrome.