PDCD1 and cancer: Immune checkpoint inhibitors (ICIs), such as cytotoxic T-lymphocyte-associated protein-4 (CTLA-4), programmed cell death-1 (PD-1), and programmed death-ligand 1 (PD-L1) inhibitors that can inhibit the co-inhibitory immune checkpoint pathways have completely changed the treatment landscape of various malignant tumors [1].