Prostate cancer has been generally viewed as an immunologically “cold” tumor (i.e., devoid of infiltrating lymphocytes), because trials evaluating T-cell checkpoint blockade therapies, including anti-PD-1 or anti-CTLA-4, have shown little benefit for all but a small number of patients with prostate cancer [1,2,3,4]. This evidence concerns the gene CTLA4 and prostate cancer.