Although the 3243A > G mutation in mt-tRNALeu(UUR) makes it a poor substrate for the MTO1–GTPBP3 complex, τm5U in MELAS patient-derived cultured cells was restored almost completely by overexpression of MTO1 (Tomoda et al. 2023); however, MTO1 overexpression led to only partial recovery of oxygen consumption, likely due to unchanged degradation of the mutant mt-tRNALeu(UUR) caused by its aberrant tertiary structure. The gene discussed is MTO1; the disease is MELAS.