Indoleamine 2,3-Dioxygenase 1 (IDO1, EC 1.13.11.52) is a cytoplasmatic hemoprotein that oxidizes tryptophan yielding N-formylkynurenine (NFK). In mammals, this reaction is the first and rate limiting step in the kynurenine catabolic pathway. IDO1 activity is low under normal physiological conditions, but is dramatically upregulated by proinflammatory cytokines such as interferon-γ. This short-term activation of IDO1 occurs as part of the innate immune response and helps to inhibit the growth of pathogens and parasites. IDO1 activation can also promote host immuno tolerance by exerting an immunosuppressive effect. IDO1 expression by tumor cells plays a susbtancial role in tumor immune tolerance, aiding tumors in evading detection and destruction. Chronic induction of IDO1 expression has been found in cancer patients increased IDO1 activity is correlated with negative prognosis. IDO1 has thus become an attractice pharmacological target for development of novel antineoplastics and adjuvants to increase the efficacy of conventional chemotherapy.