Monoclonal antibody therapy for breast cancer, illustration. Trastuzumab (yellow and green) and pertuzumab (orange blue) are monoclonal antibodies used as a combination drug to treat breast cancer, specifically the HER2 receptor in metastatic breast cancer. Here, both are shown in complex with the HER2 receptor (pink). Both drugs bind to the HER2 receptor and inhibit it, blocking its over-expresion.