Metastatic breast cancer (MBC) is an aggressive cancer for which there currently is no cure. However, there are many treatment options to help control the disease and allow patients to live longer. Despite available treatments, there is a limited understanding of how to select treatments for patients to better extend their survival. This study plans to look for changes in DNA which will help identify the treatment best suited to each patient. To achieve this, blood samples are collected from patients with MBC currently receiving treatment and changes in their DNA over time are studied to understand how these changes correlate with their response to treatment. The hope is to leverage this new dataset to develop a blood-based test which can help MBC patients select the most effective treatment.