RRM2B and mitochondrial neurogastrointestinal encephalomyopathy: In addition, patients with MNGIE-like phenotypes but mutations in genes different TP such as the ribonucleoside-diphosphate reductase subunit M2 B (RRM2B), Polymerase gamma (POLG), and mitochondrial DNA genes present with mild brain MRI abnormalities or no brain MRI abnormalities [30,31,32].