ERBB2 and breast cancer: Breast cancer is categorized into three major subtypes based on the presence or absence of molecular markers for estrogen or progesterone receptors and human epidermal growth factor 2 (ERBB2, formerly HER2): (i) hormone-receptor-positive/ERBB2-negative (70% of patients), (ii) ERBB2-positive (15–20%), and (iii) triple-negative (tumors lacking all three standard molecular markers; 15%) [2].