Over recent years, breast cancer remains the most common cancer in women in western countries along with the sharp increase in its incidence rate in developing countries. By contrast, the mortality rate has declined dramatically since the introduction of adjuvant systemic therapy, which is now used extensively for its established benefit in survival. However, a considerable number of patients experience disease progression despite optimal postoperative treatments, which hints at a demand for novel agents against specific targets with greater efficacy and lesser toxicity. Nowadays, an increasing number of targets have been yielded through various drug discovery efforts and some of these promising leads have moved beyond the stage of drug candidate, as exemplified by human epidermal growth factor receptor 2 and trastuzumab. HER2 protein, encoded by the HER2/ neu gene, belongs to a family of four homologous transmembrane receptors involved in tyrosine kinase-mediated regulation of normal breast tissue growth, differentiation, and survival. Overexpression of the HER2/neu protein, amplification of the HER2/neu gene, or both accounts for approximately 20�C25% of invasive ABT-089 dihydrochloride ductal carcinomas, which is associated with aggressive clinical history in breast cancer patients. Trastuzumab, a humanized monoclonal antibody blocking the HER2/neu receptor, is one of the targeted approaches to reach the clinic for breast cancer. Initially, trastuzumab demonstrated significant activity in patients with HER2-positive metastatic breast cancer when combined with chemotherapy. Such compelling evidence subsequently triggered the conduct of several large multicenter randomized controlled clinical trials assessing sequential or concurrent trastuzumab with chemotherapy in the adjuvant setting. These impressive results have also unequivocally revealed the striking effect of adjuvant trastuzumab on the improvement of HER2-positive breast cancer prognosis. Unfortunately, many problems are still in the air when it came to trastuzumab administration after surgery. Firstly, the definitely significant efficacy of trastuzumab was merely confined to the primary endpoint, disease-free survival, while conflicting results were MS37452 reported with regard to overall survival.