Clinically, over 90% of LHON cases are caused by one of three mtDNA missense mutations in genes encoding subunits of NADH dehydrogenase (ND): ND1 3460G>A, ND4 11778G>A and ND6 14484T>C (Wallace et al., 1988; Catarino et al., 2017; Huoponen et al., 1991). The gene discussed is MT-ND4; the disease is Leber hereditary optic neuropathy.