NUDT15*21 is assigned uncertain function based on inadequate evidence. An in vitro study expressed NUDT15*21 in HEK293 cells and found NUDT15 protein expression and thiopurine sensitivity were comparable to cells expressing NUDT15*1 (32094176). Nine out of nine variant effect predictors tested by CPIC staff predict that the NUDT15*21-defining SNV, c.410G>C (p.G137A), is neutral for protein function.