ESR1 and breast cancer: Since the role of ERβ in breast cancer remains unclear, the latest studies have made efforts to explore the relationship between ERβ and clinical outcome in many aspects, including the predictive value of ERβ expression [46], the ERβ to ERα ratio [45] and the DNA promoter hypermethylation of ERβ [47], especially in patients who have undergone endocrine therapy [48, 49] and chemotherapy [44, 50].