TPMT*40 is assigned uncertain function based on inadequate evidence. Erythrocytes from an individual genotyped TPMT*1/*40 exhibited intermediate enzymatic activity, however there was no difference in thiopurine dose intensity between TPMT*1/*1 patients and one patient genotyped TPMT*1/*40 (27564568). TPMT.40 exhibited comparable protein expression to TPMT.1 in mammalian cells (29785012). Three out of ten variant effect predictors tested by CPIC staff predict that the TPMT*40-defining SNV, c.677G>A (p.R226Q), impairs protein function.