PDCD1 and colorectal carcinoma: Immune checkpoint molecules including programmed cell death 1 (PD-1) and its ligand (PD-L1), cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), lymphocyte activation gene (LAG-3), and indoleamine 2,3-dioxygenase (IDO) have been proposed as possible targets for immunotherapy in CRC [45].