Tumor cells and immunosuppressive cells (e.g., tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs)) express enzymes like arginase 1 (Arg-1) and indoleamine 2,3 dioxygenase (IDO1), depleting arginine and tryptophan in the TME. Here, IDO2 is linked to neoplasm.