Tumor cells can release various immunosuppressive factors, such as adenylate, indoleamine 2,3-dioxygenase 1 (IDO), prostaglandin E2 (PEG2), interleukin-10 (IL-10), and transforming growth factor-β (TGF-β), to inhibit the activation of immune cells (39). This evidence concerns the gene IDO1 and neoplasm.