While initial findings from monotherapy studies involving approved CTLA-4 and PD-1 inhibitors have not demonstrated substantial benefits in unselected mCRPC patients [64,65], continued investigation guided by the identification of suitable biomarkers, rationally designed combination strategies, sequence optimization, and their application in earlier stages of the disease may yet herald clinical benefits from immune checkpoint inhibitors in prostate cancer. This evidence concerns the gene PDCD1 and prostate cancer.