TPMT*17 is assigned no function based on limited evidence. Erythrocytes from an individual genotyped TPMT*1/*17 exhibited intermediate activity (15226673). An in vitro study using a mammalian cell expression system found that TPMT.17 exhibited decreased affinity for 6-TG and enzymatic activity is decreased by 90% compared to TPMT.1 (18708949). Three out of ten variant effect predictors tested by CPIC staff predict that the TPMT*17-defining SNV, c.124C>G (p.Q42E), impairs protein function.