From PharmGKB
The thiopurines include azathioprine (a pro-drug for mercaptopurine), mercaptopurine and thioguanine. They are used to treat a variety of immunological disorders such as rheumatoid arthritis, non- Hodgkin lymphoma and ulcerative colitis. Both mercaptopurine and thioguanine can exert cytotoxic effects through the formation of thioguanine nucleotides (TGNs), active metabolites that incorporate into DNA. Mercaptopurine and thioguanine are directly inactivated by thiopurine S-methyltransferase (TPMT). Individuals with two nonfunctional TPMT alleles are at 100% risk of potentially fatal myelosuppression, due to an increased buildup of toxic TGNs. Alternative agents or a drastically reduced dose are recommended for patients with this genotype. Patients heterozygous for a nonfunctional TPMT allele are at increased risk of myelosuppression, and reduced dosing is recommended for these individuals. These dosing guidelines have been published in Clinical Pharmacology and Therapeutics by the Clinical Pharmacogenetics Implementation Consortium (CPIC) and are available on the PharmGKB website.  https://www.pharmgkb.org/chemical/PA356