The present invention provides a pharmaceutical composition for the treatment or prevention of a disease(s) caused by reduction or deficiency in 5-taurinomethyluridine (m.sup.5U) modification of mitochondrial tRNA, the composition comprising mitochondrial tRNA translation optimization 1 (MTO1) or nucleic acid encoding MTO1, wherein MTO1 is administered to a patient in an excess amount, or MTO1 is overexpressed in a patient to whom the nucleic acid has been administered.