TPMT*37 is assigned no function based on strong evidence. Erythrocytes from individuals genotyped TPMT*1/*37 exhibited intermediate activity (24710034, 33118454). The TPMT*37-defining SNV, c.648T>A (p.C216X), results in the generation of a premature stop codon and the expression of a truncated protein. The premature stop variant occurs upstream of the known "no function" variant (c.719A>G, defining TPMT*3C), suggesting that loss of this region of the protein alters function. Three out of four variant effect predictors tested by CPIC staff predict that c.648T>A (p.C216X) impairs protein function.