TPMT*19 is assigned uncertain function based on inadequate evidence. Erythrocytes from an individual genotyped TPMT*1/*19 exhibited normal activity (15652243). In an in vitro study using a yeast expression system, TPMT.19 activity was decreased by 30% relative to TPMT.1 using 6-TG as a substrate (15652243). Another in vitro study using a mammalian expression system found TPMT.19 enzymatic activity was decreased by 40% compared to TPMT.1 (18708949). Six out of ten variant effect predictors tested by CPIC staff predict that the TPMT*19-defining SNV, c.365A>C (p.K122T), impairs protein function.