Functional variants of MTCYB have previosly been associated with several human phenotypes [20]–[22], but the most compelling evidence of a prior disease association is the increased risk of developing blindness in subjects harboring the mtDNA mutations in MTND genes known to cause Leber hereditary optic neuropathy (LHON), where they synergistically interact with a primary LHON mutation to cause a defect of OXPHOS complex I activity [23]. Here, MT-CYB is linked to Leber hereditary optic neuropathy.