Herceptin is a drug used in targeted therapy for breast cancer, and can be used in breast cancer with overexpression of HER2 gene, metastatic gastric cancer or gastroesophageal junction adenocarcinoma. Herceptin, called trastuzumab in Chinese, is a monoclonal antibody prepared by transgenic technology to directly counteract the HER2 protein, and has been found to antagonize not only the growth signals in the HER2 network, but also produce antibody-dependent cell-mediated cytotoxicity, thus exerting anti-tumor effects. Either as a single agent or in combination with chemotherapy, it produces desirable clinical results and improves survival. Use is based on kilograms of body weight, with a typical initial loading dose of 4mg/kg administered intravenously over 90 minutes, and a maintenance dose of 2mg/kg weekly.This drug can be used until disease progression and is typically used for six months to one year.