ESR2 and breast cancer: Estrogen effects on breast cancer cells are mediated by ERs including ERα and ERβ, of which ERα is thought to be the main driver of ER+ breast cancer.[8] Endocrine therapy by blocking the function of ERα has become the frontline treatment option for patients with ER+ breast cancer.[5, 9, 10] This strategy aims to inhibit estrogen‐dependent cell growth by disturbing the biosynthesis of estrogen or reducing ERα activity.