TPMT*22 is assigned no function based on limited evidence. Erythrocytes from an individual genotyped TPMT*1/*22 exhibited intermediate activity (16917910). An in vitro study using a mammalian cell expression system found that TPMT.22 protein expression and enzymatic activity was undetectable (18708949). Ten out of ten variant effect predictors tested by CPIC staff predict that the TPMT*22-defining SNV, c.488G>C (p.R163P), impairs protein function.