ESR1 and breast cancer: When ER-positive/PR-negative subtype was set as reference, patients with ER-negative/PR-positive (HR, 1.18; 95% CI, 1.14-1.23) and ER-negative/PR-negative (HR, 1.27; 95% CI, 1.24-1.29) subtypes had an 18% and 27% higher risk of BC-specific death, respectively (eTable 3 in the Supplement).