MT-ND6 and Leber hereditary optic neuropathy: Interestingly, the polymorphism m.4172T>A/MT-ND1, found in family 6 mtDNA and considered a polymorphic marker of haplogroup U6a2, affects the same p.L289, causing the replacement of the non-polar leucine with the negatively charged glutamic acid, suggesting a probable synergistic role of this polymorphism in LHON pathogenesis co-occurring with the primary mutation m.14568C>T/MT-ND6.