Early results of clinical trials using checkpoint inhibitors in mCRPC patients were mixed; however, a phase II trial in 2016 demonstrated reductions in PSA in 3/10 patients and partial responses as measured by RECIST in 2/10 patients after adding pembrolizumab to standard dose enzalutamide, suggesting that PD-L1 blockade should be re-examined in prostate cancer patients [28, 29]. Here, KLK3 is linked to prostate cancer.