ERBB2 and breast cancer: Human epidermal growth factor receptor 2 (HER2) is overexpressed in 20–25% of all breast cancers,1 thus accounting for near 60,000 patients each year in the U.S.2 Use of trastuzumab to suppress HER2 activity in HER2-positive (HER2+) breast cancer opened up a new era of therapeutics in HER2+ breast cancers using monoclonal antibodies.3 However, almost all patients with HER2+ metastatic breast cancer eventually develop resistance to trastuzumab after first-line treatment; therefore, novel therapy is needed.