Trastuzumab (Herceptin; Genentech Inc.) is a humanized monoclonal antibody that binds selectively to the human epidermal growth factor receptor 2 (HER2), which is expressed in large quantities on the surface of cancer cells in some types of breast tumors. Binding of trastuzumab to HER2 can slow the growth of the cancer cells and prevent spread of the tumor. Initially investigated for use in patients with metastatic breast cancer, trastuzumab is now also being studied as an adjuvant treatment for patients with early breast cancer. Herceptin is manufactured by Genentech Inc. and contains the active substance trastuzumab. It is a humanized monoclonal antibody that binds selectively to the antigen human epidermal growth factor 2 (HER2), which is found in large amounts on the surface of some cancer cells and therefore stops growth of such cells. Herceptin is prescribed for the treatment of patients with early breast cancer who have completed chemotherapy and for the treatment of patients with metastatic breast cancer (i.e., breast cancer that has spread beyond the original tumor) who have tumors that produce large amounts of HER2. It is used either alone, in conditions where other treatments proved unsuccessful, or in combination with the chemotherapy agents paclitaxel or docetaxel as first treatment for metastatic breast cancer.