Trastuzumab is a recombinant IgG1 kappa, humanized monoclonal antibody that selectively binds with the extracellular domain of the human epidermal growth factor receptor protein (HER2). It is used as a treatment of human epidermal growth factor receptor (HER)-2+ metastatic breast cancer. It is suggested that the overexpression or gene amplification of HER2 has been found in about 20–30% of breast cancers and elevated activation of HER2 triggers multiple downstream pathways leading to abnormal proliferation of cancer cells. Trastuzumab binds to HER2 and suppresses cancer cell growth, proliferation, and survival directly and indirectly.