About 12% of U.S. women will develop invasive breast cancer over the course of her lifetime. Despite advances in early diagnosis and treatment of the disease, breast cancer remains the most commonly diagnosed non-cutaneous malignancy and the second leading cause of cancer death in American women. Acquisition of resistance to current therapies is a major challenge in everyday clinical practice, which significantly reduces the disease-free survival and overall survival in breast cancer patients. Thus, it is important to develop new therapeutic approaches for circumvention of resistance and also to identify predictive biomarkers for more effective treatment decisions. Our previous work found a protein called EZH2 as a very promising therapeutic target in metastatic breast cancer that becomes refractory to hormone therapy. Several highly selective inhibitors of EZH2 are currently being tested in phase I/II clinical trials in patients with B-cell lymphoma. In this study, we will evaluate the efficacy of these EZH2-targeting drugs in metastatic, endocrine resistant breast cancer. We further demonstrated that DNA methylation of one of EZH2-regulated target genes, called GREB1, is highly associated with EZH2 activity in advanced breast cancer. So we will test whether methylation of GREB1 can be used to identify patients who will respond to EZH2 inhibitors. Results from this clinical study provide a novel targeted therapy for advanced breast cancer and a biomarker for choosing the right treatment. Our work will pave the way for the development of personalized medicine as an alternative approach to fighting metastatic, endocrine therapy resistant breast cancer.