Immune checkpoint inhibitors which target cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) and the programmed cell death protein 1 pathway (PD-1/PD-L1) have presented substantial benefits for many types of cancers, but only a marginal benefit for prostate cancer, because of the immunosuppressive tumor microenvironment and low mutation burden [295,296]. The gene discussed is PDCD1; the disease is prostate cancer.