ESR1

ESR1 (estrogen Receptor 1), also known as estrogen receptor alpha (ERα), is a nuclear receptor encoded by the ESR1 gene on chromosome 6q25.1. As a ligand-activated transcription factor it mediates the effects of estrogen by binding estrogen response elements in target gene promoters, regulating proliferation, differentiation and survival in breast, uterus, ovary, bone and liver. estrogen binding drives a conformational change that promotes dimerization, coactivator recruitment and transcription of genes governing cell-cycle progression and apoptosis suppression; a membrane-associated pool also signals within minutes through the PI3K/Akt signaling pathway and JAK2/STAT3 signaling pathway, independently of transcription.

Roughly 70–80% of breast cancers are estrogen receptor-positive, which makes the estrogen–ESR1 axis the principal target of endocrine therapy — though the drugs attack it at two different points. Aromatase inhibitors act on the enzyme CYP19A1 to cut estrogen synthesis, starving the receptor of ligand without touching it; selective estrogen receptor modulators such as tamoxifen bind the receptor itself and act as antagonists in breast tissue while remaining partial agonists elsewhere, which is why they carry an endometrial cancer risk; and selective estrogen receptor degraders bind and destroy it.

That distinction determines what happens at resistance. Under the selective pressure of estrogen deprivation, activating mutations in the ESR1 ligand-binding domain — Y537S and D538G most often — emerge in a substantial fraction of metastatic tumors and lock the receptor in an active conformation that no longer requires estrogen. Such tumors escape aromatase inhibitors entirely, since the problem is no longer ligand supply, while remaining partly susceptible to receptor-directed degraders; the mutations are detectable in circulating tumor DNA, and an oral degrader is approved specifically for this setting in breast cancer. Beyond oncology, ESR1 is implicated in reproductive disorders including polycystic ovary syndrome and endometriosis, and in metabolic and inflammatory conditions.