Indoleamine 2,3-dioxygenase-1 (IDO1; EC 1.13.11.42) is an enzyme in the eukaryotic tryptophan catabolic pathway. It is a heme-containing, monomeric oxidoreductase that specifically catalyzes the degradation of tryptophan to N-formyl-kynurenine, which can be subsequently metabolized through a series of steps to form nicotinamide adenine dinucleotide (NAD+). IDO1 inhibition is proposed to have therapeutic potential in immunodeficiency-associated abnormalities, including cancer. Previous studies suggest that IDO may be an important regulator of the immunosuppressive mechanisms responsible for tumor escape from host immune surveillance. Several groups have demonstrated that blockade of IDO activity can directly increase the ability of tumor-bearing mice to reject tumors.