ERBB2 and breast cancer: Approximately 15–20% of invasive breast cancers (BCs) are human epidermal growth factor receptor 2-positive (HER2 +) [1], an aggressive clinical subtype [2] that, prior to the availability of HER2-targeted therapies, was associated with poor prognosis and worse survival outcomes compared with HER2-negative (HER2–) BC [1–5].