Successful recognition of tumour-associated antigens and consequent eradication of cancer by cytotoxic T lymphocytes (CTLs) can be hindered by checkpoint proteins such as cytotoxic T lymphocyte-associated protein 4 (CTLA-4) or the programmed cell death 1 (PD-1) and programmed death-ligand 1 (PD-L1) pathways. Here, PDCD1 is linked to cancer.