ERBB2 and breast cancer: Categorically, BC is divided into three major molecular subtypes based on the positive or negative expression of estrogen or progesterone receptors and human epidermal growth factor 2 (ERBB2 (or human epidermal growth factor receptor 2 (HER2)): hormone receptor-positive/ERBB2 negative (70% of patients), ERBB2 positive (15–20%), and triple negative (tumors lacking all three standard molecular markers; 15%).