The present study is designed to determine the efficacy and safety profile of the combination of lapatinib and trastuzumab (plus endocrinetherapy in ER-positive breast cancer) versus trastuzumab and chemotherapy in heavily pretreated patient population with HER2-positive MBC and to investigate the predictive role of cfDNA for detection of HER2 gene amplification on patients' outcome. The presence of circulating free DNA (cfDNA) for detection of HER2 gene amplification was associated with worse prognosis and seems to allow early response evaluation. However, many aspects of the role of cfDNA detection in patients undergoing molecular target agents such as trastuzumab or lapatinib are not well described. With the availability of improved and standardized techniques for cfDNA detection, it should now be possible to examine several of these important questions within a prospective multicenter study and a striking potential of cfDNA for detection of HER2 gene amplification might enable a more individual and optimized antimetastatic therapy inpatients with cancer.