['The authors identified a patient with mitochondrial neurogastrointestinal encephalomyopathy (MNGIE), who completely fulfilled the clinical criteria with low thymidine phosphorylase (TP) activity. However, the same homozygotic S471L TP gene mutation was also found in her unaffected mother, but with normal TP activity. To elucidate the pathogenesis of MNGIE, we performed the analysis below.', 'We analyzed the TP gene mutation in the proband and 145 unrelated individuals by direct sequence and restriction fragment length polymorphism (RFLP). TP activity was determined by the spectrophotometric method for each TP S471L genotype.', 'Among 145 normal persons, the S471L homozygote mutants were identified in 2.76% and their enzyme activity was normal.'] Is thymidine phosphorylase gene mutation a primary cause of mitochondrial neurogastrointestinal encephalomyopathy ( MNGIE )?