MT-ND4 and Leber hereditary optic neuropathy: The number of discovered mutations has increased at a fast pace since the identification of the first causal mutations in 1988, a point mutation in MT-ND4 and large mtDNA deletions for Leber hereditary optic neuropathy (LHON) and mitochondrial myopathy, respectively (Holt et al., 1988; Wallace et al., 1988).