ESR1 and breast neoplasm: The significantly higher and lower efficacy of OHT in ERα/ERβ1- and ERα/ERβ2-expressing cells, respectively, compared to cells expressing only ERα could account for the clinically better response of ERβ1-expressing ERα-positive breast tumors to endocrine therapy and, conversely, for the increased recurrence rate of ERα/ERβ2-expressing tumors [30].