TPMT*11 is assigned no function based on limited evidence. Erythrocytes from a patient genotyped TPMT*3A/*11 exhibited very low enzymatic activity and 6-MMP was undetectable in the patient's plasma and their 6-MP dose was reduced to 15% of the standard dosage (12835738). In vitro studies using mammalian cell expression systems found TPMT.11 protein expression is decreased (16220112). TPMT.11 enzymatic activity is decreased up to 97% in vitro and TPMT.11 exhibied decreased affinity for 6-MP, 6-TG, and SAM relative to TPMT.1 (16220112, 18708949). Seven out of ten variant effect predictors tested by CPIC staff predict that the TPMT*11-defining SNV, c.395G>A (p.C132Y), impairs protein function.