(2)] such as OPA1 “plus” syndrome (3), mitochondrial encephalomyopathy lactic acidosis and stroke-like syndrome (MELAS) (4), LHON/dystonia/MELAS/Leigh overlapping syndrome (5, 6), myoclonic epilepsy with ragged red fibers (MERRF) (7), POLG1-related syndrome (8), Friedreich ataxia (9), Charcot–Marie Tooth type 2A (10), Mohr–Tranebjerg syndrome (11), SPG7 syndrome (12), Wolfram syndrome (13), spinocerebellar ataxias (14), and DNMT1-related disorders (15). The gene discussed is OPA1; the disease is MERRF.