To estimate the clinical significance of estrogen receptor (ER) beta in breast cancer we reviewed some reports and compared them with our preliminary results. The structure of ERbetais similar to that of ER alpha. The DNA binding domain of ER beta is 96% conserved compared with ER alpha, and the ligand binding domain shows 58% conserved residues, suggesting that both receptors can bind estrogen responsiveelements on target genes and that they may also bind similar ligand. The targettissues of ER beta such as testis, prostate, lung, brain, thymus, and ovary, are different from those of ER alpha, ovary, uterus, endometrium, and breast. Although the function of ER beta in breast cancer progression is not well understood, 30-50% of breast cancers may express ER beta mRNA signals. Additionally, ER beta may be auseful prognostic factor in patients with breast cancer, because tumors that co-expressed ER alpha and ER beta might be node positive and tend to be of higher grade.Further characterization of the function of ER beta and its isoforms in breast cancer is warranted.