IDO1 and neoplasm: Studies examining the tumor microenvironment have shown that MSI tumors selectively increase the upregulation of immune checkpoint ligands such as PD-1 (programmed cell death 1), CTLA-4 (cytotoxic T-lymphocyte-associated protein 4), LAG-3 (lymphocyte-activation gene 3) and IDO (indoleamine 2,3-dioxygenase pathway), thereby preventing natural elimination of the tumor [31].