Metastatic breast cancer is an incurable disease in a very high percentage of 
patients. Despite new progress in endocrine and other systemic therapies, this 
evidence remains challenging for patients and clinicians. HER2 protein is a 
member of the epidermal growth factor family of transmembrane receptors. HER2 is 
overexpressed in approximately 20% to 30% of breast cancers. Overexpression of 
HER2 has been shown to be associated with increased tumor proliferation and 
relative resistance to some types of chemotherapy and hormonal therapies. 
Trastuzumab, a humanized monoclonal antibody directed against HER2 protein, has 
been shown to be an efficacious and well tolerated treatment for 
HER2-overexpressing metastatic breast cancer, both as a single agent and when it 
is used in combination with chemotherapy.