PDCD1 and cancer: Monoclonal antibodies targeting PD-1 (e.g., nivolumab, pembrolizumab, and camrelizumab), PD-L1 (e.g., atezolizumab and durvalumab), and cytotoxic T-lymphocyte antigen-4 (ipilimumab and tremelimumab) have been approved by the U.S. Food and Drug Administration (FDA) for the treatment of various cancers [53].