After controlling for age at diagnosis, race, study site, tumor stage, and histologic grade in multivariable Cox proportional hazards regression models, both percentage of tumor cell nuclei positively stained for ER (Ptrend = 0.0003) and the ER H-score (Ptrend = 0.0004) were inversely associated with breast cancer-specific mortality risk. Here, ESR1 is linked to breast cancer.