MELAS is commonly (80% of cases) caused by a A>G transition at m.3243 in MTTL1,81 but is also associated with variants in MTND5. 82 Biochemically, MELAS manifests as defects of complex I and IV activity; however, care must be taken when interpreting the findings as biochemical results can often appear normal. This evidence concerns the gene MT-TL1 and MELAS.