TPMT*25 is assigned no function based on moderate evidence. Erythrocytes from individuals genotyped TPMT*1/*25 exhibited intermediate enzymatic activity (18602085). An in vitro study using a yeast expression system found that TPMT.25 exhibited decreased affinity for 6-TG and TPMT.25 enzymatic activity is decreased by 30% compared to TPMT.1 (18602085). Seven out of ten variant effect predictors tested by CPIC staff predict that the TPMT*25-defining SNV, c.634T>C (p.C212R), impairs protein function.