TPMT*13 is assigned no function based on limited evidence. TPMT*13 was identified in an individual who experienced severe leucopenia after standard azathioprine dosing. In vitro studies found that TPMT.13 protein expression is decreased, enzymatic activity is decreased by 40-80%, and TPMT.13 exhibited decreased affinity for 6-MP, 6-TG, and SAM relative to TPMT.1 in mammalian (16220112, 18708949) and yeast cells (13679074). Six out of ten variant effect predictors tested by CPIC staff predict that the TPMT*13-defining SNV, c.83A>T (p.E28V), impairs protein function.