ER

Estrogen Receptor (ER) belongs to a superfamily of nuclear hormone receptors and is expressed in about 85% of invasive breast cancers. There are two known isoforms of estrogen receptor, ERα and ERβ. It is a weak prognostic factor but a strong predictive factor for response to endocrine therapies, both in adjuvant and metastatic settings. The primary indication to assess ER in breast cancer is to predict response to hormonal therapies such as tamoxifen, other selective estrogen receptor modulators (SERMs) and aromatase inhibitors. In univariate analysis, moderate to strong staining in even 1% of the invasive tumor cells is associated with significant improvement in disease-free survival compared to those patients whose tumor lacks ER expression.