IDO1 and neoplasm: Hypoxia and the release of various cytokines and soluble factors by tumor cells or other components of the tumor microenvironment such as transforming growth factor-β (TGFβ), IL-6, IL-10, tryptophan catabolites, prostaglandin E2 (PGE2), dickkopf-related protein 2 (DKK2), idoleamine 2,3-dioxygenase (IDO), soluble HLA-G, soluble NKG2D ligands, and galactin-3 (soluble inhibitory receptor for NKp30) have been reported to decrease the activation of NK cells and their cytotoxic activity, production of IFNγ, as well as expression and activation of its activating receptors (28–36).