TPMT*44 is assigned no function based on moderate evidence. Erythrocytes from individuals genotyped TPMT*1/*44 exhibited intermediate enzymatic activity (31005613, 32865889). Recombinant TPMT.44 expressed in E coli. exhibited decreased protein stability (31005613) and protein abundance was low when expressed in mammalian cells (29785012). Ten out of ten variant effect predictors tested by CPIC staff predict that the TPMT*44-defining SNV, c.497A>G (p.Y166C), impairs protein function.