ERBB2 and breast cancer: Breast cancer is a heterogeneous disease that can be classified into four molecular subtypes based on cell-surface receptor expression: Luminal A, Luminal B, human epidermal growth factor receptor 2 (HER2)-positive, and triple-negative breast cancer (TNBC); each of these breast cancer subtypes has distinct characteristics, epidemiology, responses to therapy, and prognoses [2,3,4].