IDO1 and cancer: In the tumor microenvironment (TME), tumor cells in harmony with cancer-associated fibroblasts and stromal cells attenuate the induced antitumor immunity by production and recruitment of a wide range of soluble and insoluble immunosuppressive molecules (such as arginase I, IDO, ROS, IL-10, TGF-β,...) and cells (such as MDSCs and regulatory T cells).