IDO1 and cancer: These cells cause immunosuppression and promote cancer progression by attenuating T-cell activity through the production of arginase 1 (ARG1), inducible nitric oxide synthase (iNOS), transforming growth factor beta (TGFβ), IL10, cyclooxygenase 2 (COX-2), indoleamine 2,3-dioxygenase (IDO), vascular endothelial growth factor (VEGF), and ROS.