Today's treatment of metastatic breast cancer is guided by characterisation of the primary tumour while 90% of deaths due to breast cancer occur as a consequence of metastases. Given important differences in molecular characteristics between metastases and primary tumour tissue, characterization of metastatic tumours instead of the primary tumour may provide better treatment guidance for patients with metastatic disease. Metastatic tumours are however difficult to assess using currently available techniques. We propose to clinically validate molecular methods and develop a clinically validated system for characterization of circulating tumour cells, used as proxies for the metastatic tumour, to guide treatment decisions in women with metastatic breast cancer. Two unique methods will be combined, single cell molecular characterization and improved circulating tumour cell sampling and identification, provided by the participating SMEs. Together with leading academic scientists and one industrial partner, the consortium is ideally suited to evaluate the clinical utility of the proposed methods and to validate the methods in order to reach the final development stage before the product will be introduced to the market, which constitutes a significant commercial potential for the SME-partners in the project.