ERBB2 and breast cancer: Breast cancer is clinically categorized into three primary subtypes based on the status of hormone receptors and human epidermal growth factor receptor 2 (HER2): luminal, which is estrogen receptor (ER)-positive and progesterone receptor (PR)-positive and further divided into luminal A and B; HER2-positive; and triple-negative breast cancer (TNBC) 4.