MT-ND4 and Leber hereditary optic neuropathy: The first discovery identified large-scale single deletions of mitochondrial DNA (mtDNA) in muscle biopsies from patients with “mitochondrial myopathies” while the second discovery identified a point mutation in the mtDNA gene for subunit 4 of NADH dehydrogenase complex (MTND4), associated with maternally inherited Leber’s hereditary optic neuropathy (LHON) in a large American pedigree.