IDO1 and cancer: Cancer cells can adopt other immunosuppressive mechanisms to create a tolerant microenvironment, such as the production of metabolic enzymes like indoleamine 2,3-dioxygenase 1 (IDO1) or arginase (ARG) or of immunosuppressive cytokines (for example interleukin (IL)-10 and transforming growth factor-β (TGF-β).