MT-ND4 and Leber hereditary optic neuropathy: In addition, many diseases caused by mutations in mitochondrial genes, or nuclear genes encoding mitochondrial proteins, primarily affect retinal ganglion cells and present as visual disorders with little or no extra-ophthalmic symptoms (e.g. MT-ND1, MT-ND4 and MT-ND6 in Leber’s hereditary optic neuropathy; OPA1 in autosomal dominant optic atrophy; Yu-Wai-Man et al., 2011; Pilz et al., 2017), suggesting retinal ganglion cell sensitivity to mitochondrial perturbations (Williams et al., 2012).