IDO1 and neoplasm: Immunosuppressive MDSCs have been implicated in tumor immune evasion through the following mechanisms [45–47]: (i) stimulating the production of ROS, which decreases T cell receptor functionality through NADPH oxidase and iNOS; (ii) producing high Arg-1 levels, which deplete T cells of L-arginine and induce cell cycle arrest; and (iii) synthesizing IDO to protect tumors from attack by specific tumor T cells by inducing tolerance through tryptophan catabolism to inhibit T cell proliferation and induce Treg cells, among others.