Utilizing CellSearch® technology, the ability to both enumerate and reliably and reproducibly characterize circulating tumor cells (CTC) for tumor markers that predict endocrine sensitivity (estrogen receptor [ER] and Bcl-2) and resistance (HER2 and Ki67) has been demonstrated. An algorithm for a CTC-Endocrine Therapy Index (CTC-ETI) has been constructed that can be calculated for each patient using the CTC enumeration and marker results. The primary goal of this study is to determine a CTC-ETI in ER positive, HER2 negative metastatic breast cancer patients before the initiation of a new endocrine therapy for the identification of patients that will progress rapidly.