A different class of mutations, with a higher pathogenic potential leading to LHON plus MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis and Stroke-like episodes) or Leigh phenotypes, have also been described in MT-ND6, MT-ND1, MT-ND3, MT-ND4 and, in particular, in the MT-ND5 genes of complex I [26]. This evidence concerns the gene MT-ND6 and Leber hereditary optic neuropathy.