ESR1 and breast cancer: In this context, studies showed that ERα- positive/ERβ1-positive tumors typically had reduced expression of Ki67 relative to ERα-positive/ERβ1-negative tumors confirming the tumor suppressor role of ERβ in the ERα-positive BC cell lines; additionally, ERβ expression diminished the pro-proliferative effects of ERα and exerted its oncosuppressive role targeting cell division [51, 52].