Conversely, some genes are most frequently associated with some specific disease known as “classic mitochondrial syndromes” such as the MT-TL1 gene mutations associated with mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), or the MT-TK mutation correlated with myoclonic epilepsy with ragged red fibers (MERRF), ND genes alterations with Leber hereditary optic neuropathy (LHON), and MT-CYB gene with exercise intolerance [23,24,25,26]. The gene discussed is MCAT; the disease is MERRF.