ERBB2 and breast cancer: Trastuzumab (Herceptin), a monoclonal antibody commonly used in the treatment of HER2+ breast cancers, is used to block the increase in HER2 signaling and the resulting activations of some related pathways, however, resistance to the drug following treatment for approximately one year is observed in the majority of HER2+ breast cancer patients (Pohlmann, Mayer, and Mernaugh 2009).