Leber's hereditary optic neuropathy (LHON) is among the most frequent mitochondrial diseases, affecting about 1 in 35 000–60 000 in Europe.1, 2 LHON is associated in over 90% of cases with one of three common mitochondrial DNA (mtDNA) point mutations affecting the Complex I subunit genes ND4 (m.11778G>A), ND1 (m.3460G>A) and ND6 (m.14484 T>C), usually occurring in homoplasmic fashion3, 4 (100% of mtDNA is mutant). This evidence concerns the gene MT-ND4 and Leber hereditary optic neuropathy.