TPMT and acute lymphoblastic leukemia: The human enzyme thiopurine S-methyltransferase (TPMT) is involved in the metabolic pathways of thiopurine drugs, such as azathiopurine (AZA) and 6-mercaptopurine (6-MP), used to treat acute lymphoblastic leukemia (ALL), inflammatory bowel diseases (IBD) and rheumatological diseases1, 2.