mt-tRNALeu(UUR)-bearing pathogenic mutations including A3243G, U3271C, or other mutations associated with MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes) lack τm5U34 (refs. 14,15), indicating that each of MELAS mutation hinders recognition by GTPBP3/MTO1, the enzymatic complex responsible for the τm5U modification16. This evidence concerns the gene MTO1 and lactic acidosis.