TPMT*26 is assigned no function based on limited evidence. An individual genotyped TPMT*1/*26 experienced hematological toxicity after standard azathioprine dosing and erythrocytes from the patient and two siblings also harboring TPMT*26 exhibited intermediate enzymatic activity (19660010). Four out of ten variant effect predictors tested by CPIC staff predict that the TPMT*26-defining SNV, c.622T>C (p.F208L), impairs protein function.