ESR1 and breast cancer: The ER, PR, and ERBB2/HER2 markers also define four major clinical subgroups of breast cancer: (1) ER-positive and HER2-negative tumors (ERpHER2n), (2) ER-positive and HER2-positive tumors (ERpHER2p), (3) ER-negative and HER2-positive (ERnHER2p), and (4) triple-negative breast cancer (TNBC, negative status for ER, PR, and HER2).