ICIs block key interactions between cancer cells and checkpoint molecules such as cytotoxic T-lymphocyte-associated protein 4 (CTLA-4, e.g., ipilimumab), programmed cell death 1 (PD-1, e.g., nivolumab), and programmed cell death ligand 1 (PD-L1, e.g., atezolizumab). This evidence concerns the gene PDCD1 and cancer.