TPMT*10 is assigned no function based on limited evidence. An individual genotyped TPMT*1/*10 was prescribed 50 mg/day 6-MP and experienced cytopenia after 3 months of treatment (10833476). In vitro studies found TPMT.10 protein expression is comparable to TPMT.1 in mammalian cells (16220112, 18708949) and yeast cells (13679074). TPMT.10 enzymatic activity is decreased by 30-70% in vitro relative to TPMT.1 (13679074, 16220112, 18708949) and TPMT.10 exhibited decreased affinity for 6-TG (18708949). Six out of ten variant effect predictors tested by CPIC staff predict that the TPMT*10-defining SNV, c.430G>C (p.G144R), impairs protein function.