TPMT*27 is assigned no function based on limited evidence. Erythrocytes from an individual genotyped TPMT*1/*27 exhibited intermediate enzymatic activity and the patient received a reduced dose of azathioprine (19945438). An in vitro study using a mammalian cell expression system found that TPMT.27 exhibited decreased protein expression that is attributed to enhanced protein degradation by the proteasome, as well as clearance by autophagy (19945438). TPMT.27 expressed in mammalian cells exhibited 90% decreased enzymatic activity compared to TPMT.1 (19945438). Molecular dynamics simulations suggest that TPMT.27 exhibited decreased protein stability (19945438). In line with these results, nine out of ten variant effect predictors tested by CPIC staff predict that the TPMT*27-defining SNV, c.319T>G (p.Y107D), impairs protein function.