The assay of enzyme activity is generally required to complement the measurement of plasma metabolite concentrations, or following the identification of novel variants of the TYMP gene, or when clinics do not have access to sequencing of TYMP. Thymidine phosphorylase activity is severely reduced in the leukocytes of patients with MNGIE, showing either no activity or activities less than 10% (0-46 nmol thymidine formed/hour/mg protein) of healthy unaffected controls (253-1000 nmol thymidine formed/hour/mg protein)[11,13]. The gene discussed is TYMP; the disease is mitochondrial neurogastrointestinal encephalomyopathy.