ERBB2 and breast cancer: Pathologic complete response (pCR), defined as no residual invasive tumor in the breast and axillary lymph nodes following neoadjuvant treatment, is associated with improved event-free and overall survival.7-11 The development of the HER2-targeted monoclonal antibody, trastuzumab (T), has led to considerably increased pCR rates and improved long-term outcomes of patients with HER2+ breast cancer.