該基因編碼通過S-腺苷-L-蛋氨酸作為S-甲基供體和S-腺苷-L-同型半胱氨酸作為副產物代謝硫嘌呤藥物的酶。硫嘌呤類藥物如6-巰基嘌呤被用作化療藥物。影響這種酶活性的遺傳多態性與個體內對此類藥物的敏感性和毒性的變化相關，從而導致硫嘌呤s-甲基轉移酶缺乏。相關假基因已在第3、18和X染色體上鑒定。
This gene encodes the enzyme that metabolizes thiopurine drugs via S-adenosyl-L-methionine as the S-methyl donor and S-adenosyl-L-homocysteine as a byproduct. Thiopurine drugs such as 6-mercaptopurine are used as chemotherapeutic agents. Genetic polymorphisms that affect this enzymatic activity are correlated with variations in sensitivity and toxicity to such drugs within individuals, causing thiopurine S-methyltransferase deficiency. Related pseudogenes have been identified on chromosomes 3, 18 and X.