NUDT15*20 is assigned uncertain function based on inadequate evidence. An in vitro study expressed NUDT15*20 in HEK293 cells and found NUDT15 protein expression and thiopurine sensitivity were comparable to cells expressing NUDT15*1 (32094176). Eight out of nine variant effect predictors tested by CPIC staff predict that the NUDT15*20-defining SNV (c.386C>G (p.P129R)) impairs protein function.