TPMT*38 is assigned no function based on limited evidence. Two patients were genotyped TPMT*1/*38. One patient with ALL received 26.9-53.8 mg/m2 mercaptopurine, which is 36-72% of the standard dose. The other patient exhibited intermediate TPMT enzymatic activity (25564374). Four out of ten variant effect predictors tested by CPIC staff predict that the TPMT*38-defining SNV, c.514T>C (p.S172P), impairs protein function.