The prognostic value of circulating tumor cells (CTC) has been demonstrated in several studies in breast cancer, especially in advanced stage [35-38]. At UCSF in the laboratory of Dr. John Park, the investigators have demonstrated expertise in the measurement of CTC in patients with advanced stage breast cancer by IC/FC assay. CTC are first enriched with immunomagnetic beads coated with EpCAM, then EpCAM+, CD45-, nucleic acid+ cells are detected by flow cytometry. This technique is highly sensitive and reproducable, and allows sorting of cells for molecular analysis as well as analysis of cell surface markers[39]. In this trial, the investigators will collect patient's peripheral blood samples every 4 weeks while on treatment and follow any reduction of CTC.