IDO1 and neoplasm: Cancer cells and the tumor microenvironment (TME) can express various signaling molecules, such as programmed death receptor 1 (PD-1), programmed death ligand 1 (PD-L1), cytotoxic T lymphocyte antigen-4 (CTLA-4), and indoleamine 2,3 oxygenase (IDO), which can help them evade immune surveillance and induce immune cell anergy [3,4,5].