ESR1 and breast carcinoma: There are over 125,000 new breast cancer diagnoses among women in the United States each year, and more than 45% of these patients are aged 65 years and older.1 For more than 2 decades, breast-conserving surgery (BCS) followed by adjuvant endocrine therapy (AET) has been a widely recommended and commonly utilized treatment approach for many patients with early-stage, estrogen receptor-positive (ER+) breast cancer.