MT-ND4 and Leber hereditary optic neuropathy: The severity of LHON varies between no symptoms to vision loss,98–100 which is caused by a gradual degeneration of the retinal nerve fiber layer resulting in optic nerve atrophy.100,101 The first pathogenic mutation identified is in the ND4 gene, a subunit of Complex I, which functions in the mitochondrial respiratory chain.102–104