ERBB2 and breast cancer: Based on the expressions of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor 2 (HER2/ERBB2), breast cancer is simply classified into three major subtypes: luminal type with hormone receptor positive/ERBB2 negative (70% of patients), ERBB2 positive (15-20%), and triple-negative (TNBC, tumors lacking all three standard molecular markers; 15%) 8, 9.