Thiopurine toxicity is associated with severe hematopoietic toxicity when patients are treated with standard doses of thiopurines, a class of antineoplastic/immunosuppressant agents that consists of mercaptopurine, thioguanine, and azathioprine. Thiopurines are prodrugs that require extensive metabolism in order to exert their cytotoxic action. Thiopurines are converted into cytotoxic thioguanine nucleotides (TG), which are incorporated into DNA and cause cell death. [From OMIM]