RRM2B analysis should also be considered in patients with mitochondrial neurogastrointestinal encephalopathy if blood/urinary deoxyuridine and thymidine levels are undetectable and thymidine phosphorylase activity is normal in white cells and platelets, based on the previous report of mitochondrial neurogastrointestinal encephalopathy secondary to compound heterozygous missense mutations in RRM2B (Shaibani et al., 2009), Case 20 reported here and because gastrointestinal symptoms were a prominent finding in our cohort of adult patients. Here, TYMP is linked to mitochondrial neurogastrointestinal encephalomyopathy.