ESR1 and breast cancer: The mainstay treatment for hormone-responsive (HR+, HER2-) breast cancer is endocrine therapy, which blocks the action of estrogen at the receptor level or inhibits estrogen production using selective ER modulation agents, aromatase inhibitors, luteinizing hormone-releasing agonists, and cyclin-dependent kinases 4 and 6 inhibitor for resistant patients (21, 22).