MT-ND4 and Leber hereditary optic neuropathy: LHON is caused by well characterized mitochondrial DNA (mtDNA) mutations, among which three so called primary mtDNA mutations account for more than 95% of all LHON cases: 11778G>A (ND4 subunit) [8], 14484T>C (ND6 subunit) [9], and 3460G>A (ND1 subunit) [10].