ERBB2 and neoplasm: We previously described that HER2 heterogeneity (defined by American Society of Clinical Oncology [ASCO]/College of American Pathologists [CAP] guidelines as the presence of 5% to 50% ERBB2-amplified cells by FISH or an area of tumor that is HER2 negative) (7) was the major determinant of pathologic complete response (pCR) to neoadjuvant T-DM1 and pertuzumab (T-DM1+P) treatment, with none of the HER2-heterogeneous cases having pCR (5).