Many cancer and tumor-associated cells in TME [e.g., tumor-associated macrophages (TAMs), tumor-associated dendritic cells (DCs), and fibroblasts] express increased indoleamine 2,3-dioxygenase (IDO) and tryptophan-2,3-dioxygenase (TDO), and large quantities of tryptophan are converted to kynurenine by the enzymes [72, 73]. Here, TDO2 is linked to cancer.