RRM2B and mitochondrial neurogastrointestinal encephalomyopathy: Defects in this gene can cause mtDNA maintenance defects, either from mtDNA depletion in RRM2B encephalomyopathic mitochondrial DNA maintenance defect (MDMD) and in RRM2B mimicking mitochondrial neurogastrointestinal encephalopathy (MNGIE) or multiple mtDNA deletions in RRM2B adPEO and RRM2B autosomal recessive PEO (arPEO) [65].