PDCD1 and colorectal carcinoma: Cytotoxic T cells are negatively regulated by the immune checkpoint molecules, cytotoxic T lymphocyte-associated protein 4 (CTLA-4) and programmed cell death protein (PD-1/PDCD-1), and PD-1 and CTLA-4 inhibitors have been approved for CRC patients with metastatic MSI tumours and are associated with prolonged survival [20].