TPMT*20 is assigned no function based on limited evidence. Erythrocytes from an individual genotyped TPMT*1/*20 exhibited intermediate activity and this patient experienced azathioprine-related pancreatitis (16917910). An in vitro study using a mammalian cell expression system found that TPMT.20 exhibited decreased affinity for 6-TG and enzymatic activity was reduced by 75% compared to TPMT.1 (18708949). Two out of ten variant effect predictors tested by CPIC staff predict that the TPMT*20-defining SNV, c.712A>G (p.K238E), impairs protein function.