MT-TL1 and MELAS: For example, about 80% of patients with MELAS harbor the m.3243A>G mutation in the mitochondrial tRNALeu(UUR) gene (MTTL1), whereas mutations in polypeptide-coding genes, including subunits 1, 5, and 6 of complex I (MT-ND1, MT-ND5, and MT-ND6), have also been identified in MELAS (1, 5).