Thiopurine methyltransferase (encoded by TPMT) and nudix hydrolase 15 (encoded by NUDT15) play important roles in the metabolism of the thiopurines mercaptopurine and thioguanine, which are widely used in the treatment of acute lymphoblastic leukemia, inflammatory bowel diseases and autoimmune disorders. The gene discussed is TPMT; the disease is Autoimmunity.