TPMT*24 is assigned uncertain function based on inadequate evidence. Erythrocytes from individuals genotyped TPMT*1/*24 exhibited intermediate activity (18602085, 27564568), while erythrocytes from individuals genotyped TPMT*3A/*24 or *3C/*24 exhibited low enzymatic activity (33118454). There is no difference in thiopurine dose intensity observed between individuals genotyped TPMT*1/*1 and TPMT*1/*24 (27564568, 34412101). An in vitro study using a yeast expression system found that TPMT.24 exhibited decreased affinity for 6-TG, but the rate of enzymatic activity is unaffected compared to TPMT.1 (18602085). TPMT.24 exhibited comparable protein expression to TPMT.1 in mammalian cells (29785012). One out of ten variant effect predictors tested by CPIC staff predicts that the TPMT*24-defining SNV, c.537G>T (p.Q179H), impairs protein function.