Detection of TPMT genetic variants by genotyping is recommended prior to commencing the treatment of patients with Thiopurine drugs such as Azathioprine, 6-Mercaptopurine and 6-Thioguanine for Acute Lymphoblastic Leukemia ; autoimmune disorders like Crohns disease ; Rheumatoid arthritis.TPMT is an enzyme which converts these drugs in the body to an active form. Any genetic defect in this enzyme predisposes patients to a heightened risk of drug-induced bone marrow toxicity due to accumulation of unmetabolised drug. Approximately 0.3% of individuals lack TPMT activity while 11% show reduced activity (variants TPMT*2, 3A ; 3C). 89% of people have normal activity (variant TPMT*1).