Is oestrogen receptor- β a predictor of endocrine therapy responsiveness in human breast cancer?

The role of oestrogen receptor (ER) β in human breast cancer remains unclear. However, it is now apparent that when considering ER β in human breast cancer it is important to recognise two ER β expressing groups, one in which ER β is co-expressed with ER α and the other where ERβ is expressed alone. Emerging data support different functions between ER β when it is expressed alone and when it is co-expressed with ER α. With regard to the latter group (ER α +/ER β +), there are now 9 out of 10 retrospective clinical outcome studies published, that support the hypothesis that increased expression of ER β is associated with increased likelihood of response to endocrine therapy. The data strongly support undertaking prospective studies to determine if the addition of ERβ to ER α is clinically beneficial and whether to include both ER β and ER α when establishing clinically relevant cut-offs for defining ER status.