TPMT*18 is assigned no function based on limited evidence. Erythrocytes from an individual genotyped TPMT*1/*18 exhibited intermediate activity (15226673). An in vitro study using a mammalian cell expression system found that TPMT.18 protein expression is decreased compared to TPMT.1 and TPMT.18 enzymatic activity is undetectable (18708949). Ten out of ten variant effect predictors tested by CPIC staff predict that the TPMT*18-defining SNV, c.211G>A (p.G71R) impairs protein function.