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Indoleamine 2,3-dioxygenase (IDO) is an enzyme catalyzing the oxidative degradation of L-tryptophan in the kynurenine pathway, in which the pyrrole ring of L-tryptophan is cleaved to generate N-formyl-kynurenine. Mature human IDO enzyme is a 45 kDa monomeric protein of 403 amino acids that is encoded by the IDO gene (15 kb with 10 exons). According to X-ray crystallographic analysis, the chemical structure of human IDO enzyme consists of two distinct α-helical domains (one small and one large) and a heme prosthetic group, where the heme is coordinated to the active site by a histidine (His) imidazole as the proximal fifth ligand.