PDCD1 and cancer: Monoclonal antibodies targeting CTLA-4 (ipilimumab), PD-1 (pembrolizumab, nivolumab, and cemiplimab) or its associated programmed cell death ligand 1 (PD-L1, atezolimumab, durvalumab and avelumab), known as checkpoint inhibitors (ICIs), have been approved by the FDA for the treatment of various types of cancer [660].