TPMT*39 is assigned no function based on limited evidence. Erythrocytes from an individual genotyped TPMT*1/*39 exhibited intermediate enzymatic activity and the patient was started on a reduced dose of thiopurines but continued to experience side effects (27703193). TPMT.39 expressed in mammalian cells exhibited decreased protein abundance relative to TPMT.1 (29785012) Eight out of ten variant effect predictors tested by CPIC staff predict that the TPMT*39-defining SNV, c.218C>T (p.A73V), impairs protein function.