TPMT*41 is assigned no function based on limited evidence. Erythrocytes from an individual genotyped TPMT*1/*41 exhibited intermediate enzymatic activity (28462921, 32865889). In vitro studies using bacterial expression systems found that TPMT.41 exhibited decreased protein stability, decreased affinity for SAM, and decreased enzymatic activity compared to TPMT.1 (28462921, 32865889). Nine out of ten variant effect predictors tested by CPIC staff predict that the TPMT*41-defining SNV, c.719A>C (p.Y240S), impairs protein function.