ERBB2 and breast cancer: Breast cancer is frequently classified into four molecular subtypes: (i) luminal A subtype, which has high estrogen and progesterone receptor expression but low human epidermal growth factor receptor 2 (HER2) expression; (ii) luminal B subtype, which has lower estrogen and progesterone receptor levels and variable HER2 expression; (iii) basal-like triple negative subtype, which has negative estrogen receptor, progesterone receptor, and HER2 expression; and (iv) HER2 subtype, which has high HER2 expression but generally does not express estrogen and progesterone receptors (9).