In population-based studies, human epidermal growth factor receptor 2 (HER2) is overexpressed in ~13–22% of all patients with breast cancer, frequently as a consequence of amplification of the ERBB2 gene (i.e., clinical “HER2+”)1–3, and confers aggressive clinical behavior (particularly a more rapid rate of cell proliferation and propensity for metastases), resulting in poor clinical prognosis1–5. Here, ERBB2 is linked to breast cancer.