ERBB2 and breast cancer: HER2 is amplified in up to 20% of primary breast cancers and is associated with poor patient prognosis.1 Monoclonal antibodies that target HER2, such as trastuzumab and pertuzumab, as well as small molecule kinase inhibitors, such as lapatinib and neratinib, which target both HER2 and EGFR, have been approved by the Food and Drug Administration (FDA) for the treatment of HER2-positive cancer.2 However, despite initial responses to these agents, acquired resistance almost universally develops.3