ESR1 and breast cancer: In the clinical setting, breast cancer is currently categorized into four subtypes: luminal A and B (estrogen receptor (ER) and/or progesterone receptor (PR) positive, human epidermal growth factor receptor 2 (ERBB2, HER2) negative), HER2 subtype (HER2+, ER-, PR-), and triple negative (TN; HER2-, ER-, PR-) breast cancer.