MT-ATP6 and maternally-inherited Leigh syndrome: The most frequent mutations in the MT-ATP6 gene affect the same amino acid residue of subunit a, p.L156R (m.8993T>G) and p.L156P (m.8993T>C) and have been identified in numerous patients who present the NARP (neuropathy, ataxia, and retinitis pigmentosa) or MILS (maternally inherited Leigh syndrome) syndromes, depending on the mutation load.