The populations of immunosuppressive cells found in immunologically “cold” tumors promote a tolerogenic environment via the release of cytokines and molecules (e.g., IL-10, TGF-β, indoleamine 2,3-dioxygenase (IDO) [213], and CXCL12 [214]) inhibiting tumor cell clearance by immune effectors. This evidence concerns the gene IDO1 and neoplasm.