Metastatic breast cancer is incurable and is the second most common cause of cancer death for women in the United States. Current treatment approaches tend to relieve symptoms without curing the disease and typically involve multimodal therapy that combines hormonal, chemo-, and radiation therapies. Treatment is modified when it provides limited or no benefit, when side effects are severe, or when signs of disease progression appear. Current methods for evaluating response to treatment and detecting disease progression, such as physical examination, radiological imaging studies, or blood tests for tumor markers, are largely unreliable. Consequently, patients may undergo several cycles of treatment before their clinical response can be determined or may continue to receive treatment that has become ineffective. Detection of tumor cells in bone marrow has been shown to be a direct indication of metastasis and bone marrow testing is frequently used to identify patients with poor prognosis. Disadvantages of bone marrow sampling are that it is invasive and painful, and repeated testing is not convenient and may have a negative impact on patients. Analysis of a blood sample for circulating tumor cells (CTCs) has been proposed as a noninvasive method for breast cancer monitoring.