ESR1 and breast cancer: In our cohort of early stage, ER-positive (ER+), HER2-negative (HER2−) lymph-node-negative (LN−) breast cancer patients, a multivariate Cox proportional hazards model was used to investigate the effect of age, stage, grade, tumor subtype, race, Oncotype DX score classification, and chemotherapy status on breast-cancer-specific survival (BCSS).