4. Candidate for abiraterone therapy with documented evidence of metastatic castration-resistant prostate cancer. Metastatic status is defined as at least one documented metastatic lesion on either bone scan or CT/MRI scan. Castration resistant prostate cancer is defined as rising PSA or other signs of disease progression despite treatment with androgen deprivation therapy and the presence of a castrate level of testosterone (≤50 ng/dL).