IDO1 and neoplasm: These include the lack of tumor-specific antigens that are not expressed in primary epithelium (18), the local suppressive tumor microenvironment [characterized by expression of IL10, TGFβ, IDO, COX-2, adenosine, and arginase, and presence of regulatory T-cells and myeloid-derived suppressor cells (19, 20)], and the physical barriers that trap immune cells in the stroma [also called immune exclusion, (21, 22)].