ESR1 and breast cancer: In estrogen receptor positive (ER+) early-stage breast cancer, 5 years of adjuvant endocrine therapy substantially reduce the risks of locoregional and distant recurrence, contralateral breast cancer, and death.[3–5] Tamoxifen, a selective estrogen receptor (ER) modulator (SERM), is the most frequently used antiestrogen adjuvant treatment for ER+ pre-menopausal women.