PDCD1 and neoplasm: During past decades, the application of T-cell immune checkpoint inhibitors (ICIs) such as cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) and programmed cell death protein 1 (PD-1)/programmed cell death-ligand 1 (PD-L1) monoclonal antibodies have completely opened the door to tumor immunotherapy (12–16).