Breast cancer spreads to distant sites, including bone, liver and brain, primarily by metastasis through the bloodstream. While most cancer deaths are caused by such blood-borne dissemination of the primary tumor, metastasis is not readily studied in humans, since cancer cells are extremely rare in the blood (estimated at one tumor cell per billion normal blood cells) and there are very significant technical hurdles that complicate their isolation and analysis. Dr. Haber's research team has recently developed a new microfluidic approach to the isolation of circulating tumor cells (called CTCs), which offers improved capture and subsequent molecular analysis of these rare cells. Using their "CTC-Chip" for isolation of tumor cells in the bloodstream of patients with breast cancer, Dr. Haber and his colleagues propose to study whether certain molecular biomarkers within CTCs can provide an early sign of response to hormonal therapy. This could allow determining, within a few days, whether a tumor is responding to the given therapy and either continue or switch therapies faster. The investigators will also develop new approaches to study the types of genes that are expressed in these cells, since they are likely to provide key information about the process by which tumor cells enter the bloodstream. Such studies may one day provide new tools to monitor and ultimately prevent breast cancer metastasis.