ERBB2 and breast cancer: Hormone receptor (HR)-positive breast cancer, including HR-positive, human epidermal growth factor receptor 2 (HER2)-negative (HR+/HER2−) and HR-positive, HER2-positive (HR+/HER2+) breast cancer, accounts for a large proportion of breast cancer cases; indeed, HR+/HER2− breast cancer is the most common subtype1, 2.