To test the hypothesis that HDAC inhibition may enhance the efficacy of ICIs in the clinic, we have conducted a phase I/Ib, open label, dose finding study of the combination of the Class I/II HDAC inhibitor vorinostat and the PD‐1 inhibitor pembrolizumab in patients with advanced RCC, urothelial and prostate cancer. Here, HDAC9 is linked to prostate cancer.