TPMT*28 is assigned no function based on limited evidence. Erythrocytes from an individual genotyped TPMT*3A/*28 exhibited very low enzymatic activity (20945351). The patient exhibited a plasma concentration of 6-TGN above the therapeutic range and MMP was undetectable (20945351). Seven out of ten variant effect predictors tested by CPIC staff predict that the TPMT*28-defining SNV, c.349G>C (p.G117R), impairs protein function.