TPMT*5 is assigned no function based on limited evidence. Erythrocytes from an individual genotyped TPMT*1/*5 exhibited intermediate enzymatic activity (9246020). In vitro studies using mammalian cell expression systems found that TPMT.5 protein expression is decreased and TPMT.5 is degraded more rapidly compared to TPMT.1 (16220112). Additionally, TPMT.5 enzymatic activity is undetectable in vitro (16220112, 18708949). Nine out of ten variant effect predictors tested by CPIC staff predict that the TPMT*5-defining SNV, c.146T>C (p.L49S), impairs protein function.