RRM2B and Kearns-Sayre syndrome: Mutations in the RRM2B gene encoding p53-inducible ribonucleoside reductase subunit 2B caused severe infantile disease, with hypotonia, tubulopathy, seizures, respiratory distress, diarrhea, and lactic acidosis [26] but application of whole exome sequencing (WES) has revealed unusual late-onset cases with PEO [27] or KSS [28], and a case with mitochondrial neurogastrointestinal encephalomyopathy (MNGIE syndrome) [29].