TPMT*1 is assigned normal function based on definitive evidence. TPMT*1 corresponds to the reference sequence, NG_008807.2, containing no SNVs. As such, TPMT.1 activity is used as the reference for in vitro studies using yeast and mammalian cell expression systems (8561894, 9177237, 13679074, 16220112, 18708949). Erythrocytes from individuals genotyped TPMT*1/*1 exhibited normal to high enzymatic activity (9246020, 15226673, 23252716, 31005613). Lastly, TPMT*1/*1 patients have been shown to tolerate standard thiopurine dosing (23585236, 29387964, 33928426, 27564568, 11304783, 10580024, 19057372).