TPMT*23 is assigned no function based on limited evidence. Erythrocytes from individuals genotyped TPMT*3A/*23 or *3C/*23 exhibited low activity (17885628). An in vitro study using a mammalian cell expression system found that TPMT.23 exhibited decreased affinity for 6-TG and TPMT.23 enzymatic activity is decreased by 40% compared to TPMT.1 (18708949). Published in silico modeling suggests that TPMT.23 exhibited decreased protein stability as well as altered substrate and co-factor binding (17885628, 23025308). Similarly, seven out of ten variant effect predictors tested by CPIC staff predict that the TPMT*23-defining SNV, c.500C>G (p.A167G), impairs protein function.