ESR1 and breast carcinoma: Breast cancer is the most common malignancy among women, with 67%-81% of cases being estrogen receptor positive (ER+).1 For decades, tamoxifen has been the gold standard for endocrine treatment of all stages of ER + breast cancer.2 The therapy lasts initially 5 years, with an option to extend it as adjuvant therapy (EAT) for another 5 years, or until a recurrence occurs.