They include IDO, PD-L1, tryptophan 2,3-dioxygenase (TDO), arginase 1 and 2, CCL22, TGF β, and FoxP3, which are expressed by heterogeneous cell types in the tumor microenvironment, such as MDSCs, tumor-associated macrophages (TAMs), tumor-associated neutrophils (TANs), tumor-associated dendritic cells (DCs), Treg cells and cancer-associated fibroblasts (CAFs), that help tumor cells escape immune cell attack (Fig. 3) [93]. Here, TDO2 is linked to cancer.