PDCD1 and cancer: Several types of immune checkpoint inhibitors for cancer treatment have been approved recently—anti-PD-1 monoclonal antibodies (such as pembrolizumab and nivolumab); anti-PD-L1 monoclonal antibodies (such as atezolizumab); and CTLA-4 monoclonal antibodies (such as ipilimumab, avelumab, and durmalumab) [31].