An evaluation of thymidine phosphorylase activity is typically required to complement the measurement of thymidine and deoxyuridine concentrations in body fluids, or upon the identification of novel variants of the TYMP gene, or when clinics do not have access to Sanger sequencing of TYMP. Thymidine phosphorylase activity in the leukocytes of patients with MNGIE are severely reduced, showing little (<10% of healthy unaffected controls) or no activity (Spinazzola et al., 2002; Martí et al., 2004). The gene discussed is TYMP; the disease is mitochondrial neurogastrointestinal encephalomyopathy.