TPMT*42 is assigned no function based on strong evidence. Erythrocytes from individuals genotyped TPMT*1/*42 exhibited intermediate enzymatic activity (31005613, 32865889), while erythrocyte from an individual genotyped TPMT*3A/*42 exhibited low enzymatic activity (31005613, 32865889).The TPMT*42-defining SNV, c.95_96insA, results in a frameshift of the coding sequence and the creation of a premature stop codo, which is predicted to undergo nonsense-mediated decay. The variant effect predictors, CADD and PhD-SNPg, predict that c.95_96insA (p.Trp33ValfsTer26) impairs protein function.