Despite this highly inflamed environment, MSI-H tumors persist in such hostile climate due to overexpression of immune checkpoint molecules as cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), lymphocyte-activation gene 3 (LAG-3), PD-1/PD-L1, and IDO, hampering an efficient anti-tumor T-cell response (87, 114, 115). Here, IDO1 is linked to neoplasm.