 Trastuzumab, a humanized monoclonal antibody against the extracellular domain of HER2, inhibits the growth of breast tumor cells by receptor internalization, inhibition of cell-cycle progression and recruitment of immune-effector cells. Trastuzumab, as the first anti-HER2 therapy, is approved since 1998 in the United States and since 2000 in European countries for the treatment of HER2-overexpressing metastatic breast cancer MBC. Single therapy as well as combination with chemotherapy has significantly improved the outcome of these women. Shown by longer time to disease progression, longer duration of response, lower rate of death at 1 year, longer survival and a 20 per cent reduction in the risk of death. 