TPMT*9 is assigned uncertain function based on inadequate evidence. Clinically, erythrocytes from individuals genotyped TPMT*1/*9 exhibited intermediate activity (15226673, 18602085, 23252716, 30840585, 31005613). However, one individual genotyped TPMT*1/*9 exhibited normal TPMT enzymatic activity (15226673). An individual genotyped TPMT*1/*9 received 67 mg/m2 6-MP and exhibited a 6-TGN concentration of 502 pmol, which is above the therapeutic range (23252716). In vitro studies using mammalian expression systems found that TPMT.9 protein expression is comparable to TPMT.1 (16220112, 18708949). However, one study found that TPMT.9 exhibited normal enzymatic activity at ~92% of TPMT.1 activity using 6-MP as a substrate (16220112), while another found that enzymatic activity was decreased by 25% using 6-TG as a substrate (18708949). Similarly, TPMT.9 expressed in yeast cells exhibited a 25% decrease in enzymatic activity compared to TPMT.1, as well as decreased affinity for 6-TG (18602085). Molecular modeling suggests that TPMT.9 exhibited decreased protein stability (18602085). Five out of ten variant effect predictors tested by CPIC staff predict that the TPMT*9-defining SNV, c.356A>C (p.K119T), impairs protein function.