Since their invention [1], thiopurines (azathioprine, 6-mercaptopurine [6MP], and 6-thioguanine [6TG]) have significantly advanced the treatment of acute lymphoblastic leukemia, organ transplantation, inflammatory bowel disease (IBD) and various autoimmune diseases. These drugs are often regarded as the best model for a pharmaco genomic approach to drug dosing, which has led to a plethora of candidate genes being examined for their pharmacogenomic potential in modifying the prescription of thiopurines. These genes affect thiopurine response by acting on the metabolism, transport and receptor/effector functions of the drugs, and are likely to remain the focus of investigations in the foreseeable future.