PDCD1 and cancer: Immune checkpoint inhibitors (ICIs) are monoclonal antibodies that target certain immune checkpoints, such as cytotoxic T lymphocyte-associated protein 4 (CTLA-4), programmed cell death protein 1 (PD-1), and lymphocyte activation gene - 3 (LAG-3), which play a role in promoting T-cell activation and antitumor effects and have become the therapeutic treatment for many malignant tumors (1, 2).