Female patients 18 years or older with estrogen receptor (ER) positive, HER2 negative, progressive metastatic breast cancer (MBC) after one or more lines of endocrine therapy (ET) who are initiating a new ET will be enrolled into the study. Patients must have immunohistochemistry (IHC) proven ER positive disease, IHC and/or fluorescence in-situ hybridization (FISH) proven HER2 negative disease, and an ECOG performance status of 0-2. Patients with brain metastases only or those who are progressing on fulvestrant are not eligible for the study.