ERBB2 and breast cancer: The prognosis for locally advanced or metastatic breast cancer (LA/mBC) is poor, having an estimated 5-year survival rate of approximately 29.0% with distant metastasis.1,2 Treatment decision-making is typically informed by molecular characteristics including hormone receptor (HR) status, human epidermal growth factor receptor 2 (HER2) expression, and breast cancer susceptibility gene (BRCA) mutation status.