Estrogen receptor (ER)

The estrogen receptor (ER) is a nuclear hormone receptor and ligand-activated transcription factor found in many tissues, including breast tissue. In breast cancer, ER-positive tumors are defined by the presence of estrogen receptors on tumor cell nuclei and depend on estrogen for proliferation and growth. These receptors bind estrogen, inducing a conformational change that allows them to regulate the transcription of target genes involved in cell proliferation and survival. Roughly 70-80% of all breast cancers are ER-positive, making ER the most common and extensively validated therapeutic target in breast cancer. Therapeutic inhibitors—either by blocking the receptor (SERMs, SERDs) or reducing estrogen production (aromatase inhibitors)—can effectively limit tumor growth. The testing for ER expression is critical in breast cancer diagnosis, prognosis, and treatment planning, and is usually performed using immunohistochemistry on tumor tissue. ER status also guides the use of hormone therapies and predicts the benefit from these treatments.

Other names

Estrogen receptorEREstrogen receptor alpha (ERα)Estrogen receptor beta (ERβ)ESR1Hormone receptor

Selective estrogen receptor modulators (SERMs): bind to ER, block estrogen signaling in breast tissue (e.g., tamoxifen). Selective estrogen receptor degraders (SERDs): promote degradation of the estrogen receptor (e.g., fulvestrant). Aromatase inhibitors: decrease estrogen synthesis, lowering ligand availability for the receptor.