MTHFR and Kearns-Sayre syndrome: We included a pediatric patient with KSS who was treated with FA and subsequently with folinic acid at Osaka City General Hospital, and an adult patient homozygous for MTHFR C677T polymorphism (rs1801133, NM_005957.5:c.665C>T, NP_005948.3:p.Ala222Val) who was treated with FA at National Hospital Organization Nishiniigata Chuo Hospital.