ERBB2 and breast cancer: Breast cancer is a highly heterogeneous disease, categorized into four subtypes based on hormone receptor and human epidermal growth factor receptor 2 (HER2) status: luminal A (HR+/HER2-), luminal B (HR+/HER2+), HER2-positive (HR-/HER2+), and triple-negative breast cancer (TNBC, HR-/HER2-) [1].