MT-ND6 and Leber hereditary optic neuropathy: The mtDNA background, as defined by the genetic variability of haplogroups, has been consistently implicated in LHON penetrance, in particular for the two mutations not affecting complex I redox activity, m.11778G > A/MT-ND4 and m.14484T > C/MT-ND6 mutations [42,43].