ICIs are monoclonal antibodies (mAbs) that block inhibitory molecules involved in regulation of immune system pathways, such as cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) (e.g., ipilimumab), programmed cell death protein 1 (PD-1) (e.g., nivolumab, pembrolizumab, cemiplimab), or its ligand programmed cell death protein ligand 1 (PD-L1) (e.g., atezolizumab, avelumab, durvalumab) in the tumor microenvironment, which leads to systemic immune cell activation (Figure 1) (6, 7). The gene discussed is PDCD1; the disease is neoplasm.