Finally, RRM2B screening should be considered in patients with mitochondrial neurogastrointestinal encephalopathy when deoxyuridine and thymidine levels in both blood and urine are negative, thymidine phosphorylase activity is normal in white cells and platelets and analysis of the TYMP gene does not identify causative mutations. The gene discussed is RRM2B; the disease is mitochondrial neurogastrointestinal encephalomyopathy.