IDO1 and neoplasm: Other factors that contribute to tumor cell escape include 1) developing an immunosuppressive TME by producing immunosuppressive cytokines such as vesicular endothelial growth factor (VEGF), transforming growth factor β (TGF‐β), or indoleamine 2,3‐dioxygenase (IDO); 2) recruiting regulatory immune cells that function as the effectors of immunosuppression and; 3) expressing inhibitory co‐stimulatory molecules (such as CTLA‐4, PD1, PD‐L1).