ICIs are monoclonal antibodies that target immune checkpoints (e.g., cytotoxic T-lymphocyte antigen-associated protein 4 [CTLA-4], programmed cell death-1 [PD-1], programmed cell death ligand 1 [PD-L1]) on T-, cancer and antigen-presenting cells (Figure 2) [23]. The gene discussed is PDCD1; the disease is cancer.