PDCD1 and cancer: ICIs such as the inhibitors of cytotoxic T-lymphocyte antigen-4 (CTLA-4; ipilimumab), programmed cell death receptor-1 (PD-1; nivolumab, pembrolizumab), and its ligand (PD-L1; atezolimumab, avelumab, and durvalumab) have revolutionized cancer treatment (2).