Expression and activity of IDO on tumor and immune cells can be modulated by several signaling systems, including the engagement of toll-like receptors (TLRs), tumor necrosis factor superfamily members (TNFRs), interferon beta receptor (IFNBR), interferon gamma receptor (IFNGR), and transforming growth factor beta receptors (TGFBRs), which are able to induce or maintain IDO expression. Here, IDO1 is linked to neoplasm.