IDO2 and neoplasm: One of the strongest mediators of the tumor-induced immunosuppression is kynurenine, the product of tryptophan catabolism via tryptophan dioxygenase (TDO) [8] and indoleamine 2,3-dioxygenase enzymes (IDO1 and IDO2) [9], which are induced by interferon- γ (IFN-γ) [10, 11], nitric oxide (NO) [12] and iron [13].