This research study is evaluating a drug called enzalutamide in metastatatic castration resistant prostate cancer.Enzalutamide is already FDA approved for metastatic castration resistant prostate cancer after treatment with chemotherapy. The purpose of this study is to analyze features of tumor specimens sampled prior to therapy and at disease progression to determine why patients respond or stop responding to treatment with Enzalutamide. Prior chemotherapy is not a requirement of this trial. Enzalutamide is a drug designed to block the effects of male hormones in the body that may be helping prostate cancer to grow. It has already been approved by the FDA (the U.S. Food and Drug Administration) for patients with castration-resistant prostate cancer who have received prior chemotherapy. Additionally, this study will analyze features of tumor specimens sampled prior to therapy and at disease progression to determine why patients respond or stop responding to treatment with enzalutamide.