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The hormone switch that drives most breast cancers. Blocking or destroying it is the oldest and most effective targeted therapy.
Tamoxifen (SERM), aromatase inhibitors, fulvestrant (SERD), oral SERDs (elacestrant, imlunestrant, camizestrant), and the PROTAC degrader vepdegestrant (approved 2026 for ESR1-mutant disease) form the endocrine armamentarium. ESR1 mutations arise under aromatase-inhibitor pressure and are detected by ctDNA.
In plain words · The hormone switch that drives most breast cancers. Blocking or destroying it is the oldest and most effective targeted therapy.
The hormone switch that drives most breast cancers. Blocking or destroying it is the oldest and most effective targeted therapy.
Ligand-activated nuclear receptor; ESR1 Y537S/D538G mutations render it ligand-independent.
17 products aim at Estrogen receptor (ERα): small molecules, degraders, hormonal therapies, imaging agents and other agents. Drugs cut off the hormone supply, block the receptor so the hormone cannot bind, or send the receptor to the cell's waste disposal.
Tumour-associated overexpression: HPA finds the RNA cancer enhanced in cancer (Breast Invasive Carcinoma (TCGA), Uterine Corpus Endometrial Carcinoma (TCGA)) and tissue enhanced in normal cervix, endometrium 1, fallopian tube, so the tumour and the normal tissue it comes from share the target and the medicine relies on the difference in level. HPA ESR1: RNA tissue enhanced (cervix 81 nTPM, endometrium 1 104 nTPM, fallopian tube 61 nTPM); high antibody staining in 3 normal tissues; highest cancer staining breast cancer (7 of 10 high). Distribution: 2 cancer families in the corpus carry a prevalence row, label threshold or catalogue link for it (Breast cancer (all types), Endometrial cancer); approvals of single-target medicines aimed at it also list Ovarian cancer, not counted; Open Targets associates it with 4 specific cancer types at or above 0.5 (breast cancer, breast carcinoma, breast adenocarcinoma, prostate cancer). (Rule 7 of scripts/fetch-target-specificity.ts.)
Sources: Human Protein Atlas ESR1 tissue; Human Protein Atlas ESR1 pathology; Open Targets ENSG00000091831 associations
First described 1986. Earliest sequence paper UniProt cites for the protein: Green et al, Nature, 1986, "Human oestrogen receptor cDNA: sequence, expression and homology to v-erb-A". Source.
What a pathology or genomic report can say about this target, each with the thresholds approvals use.
Cell lines and mouse models for this target →
Ligand-activated nuclear receptor; ESR1 Y537S/D538G mutations render it ligand-independent.
RNA: tissue enhanced (cervix 81 nTPM, endometrium 1 104 nTPM, fallopian tube 61 nTPM), detected in many normal tissues.
Medium: Breast, Efferent ducts, Fallopian tube.
RNA cancer enhanced: Breast Invasive Carcinoma 132 pTPM, Uterine Corpus Endometrial Carcinoma 58 pTPM.
HPA ESR1 tissue · HPA ESR1 pathology · HPA protein class: FDA approved drug targets
Human Protein Atlas version 25.1, antibody staining at reliability approved, enhanced or supported; used under CC BY-SA 3.0. Staining counts are patients per level in the atlas cohort, not population prevalence.
| Cancer | Prevalence | Measure | Note | Source |
|---|---|---|---|---|
| HR-positive / HER2-negative breast cancer | 100% | ER+ (>=1% IHC), defining | ~70% of all breast cancers; ESR1 mutation ~30% after AI | Wikipedia |
| Endometrial cancer | 70-80% | ER expression (endometrioid) | Wikipedia |
Approximate, population-level figures; the measure column says what was counted. Ranges show the midpoint as a bar.
Camizestrant is an oral oestrogen-receptor degrader approved in September 2026 for a new kind of decision: switching treatment when a blood test shows resistance developing, before the cancer visibly grows.
Diethylstilbestrol was the first hormonal treatment for prostate cancer, standard from the 1940s, and was also given for breast cancer, before its cardiovascular harms and the cancers it caused in the daughters of women who took it in pregnancy ended its use.
Elacestrant was the first oral oestrogen-receptor degrader (2023), for ESR1-mutant breast cancer detected by blood test.
Exemestane is a steroidal aromatase inhibitor, the partner of everolimus and the agent tested with ovarian suppression in young women.
A PET scan that shows which breast cancer deposits still have oestrogen receptors, helping decide whether hormone therapy will work when biopsy is impractical.
Fulvestrant is a monthly intramuscular injection that degrades the oestrogen receptor rather than blocking it, the endocrine backbone paired with CDK4/6, PI3K and AKT inhibitors once aromatase inhibitors fail. Slow uptake and incomplete receptor degradation drove the search for oral degraders.
Giredestrant is Roche's oral SERD: it failed to beat an aromatase inhibitor in first-line metastatic disease but succeeded after surgery and after CDK4/6 failure.
Monthly or 3-monthly injections that switch off the ovaries, letting premenopausal women use aromatase inhibitors and lowering recurrence in higher-risk cases.
A blood test that reads a tumour's mutations without a tissue biopsy and is the FDA-approved gateway to several targeted drugs.
Imlunestrant is Lilly's oral oestrogen-receptor degrader, approved in 2025 for ESR1-mutant breast cancer and shown to work with abemaciclib regardless of mutation.
Daily pills that stop the body making oestrogen after menopause, the backbone of hormone therapy for most breast cancers.
LNS8801 is an experimental investigational agent whose form is not stated in the registry from Linnaeus Therapeutics in phase 3 trials for melanoma, aimed at Estrogen receptor (ERα).
Progesterone-like hormones that can reverse early endometrial cancer in women who want to keep their uterus, and control advanced hormone-sensitive disease.
Raloxifene (Evista) is an osteoporosis tablet that also lowers the chance of developing invasive breast cancer in postmenopausal women at higher risk, with fewer womb-cancer problems than tamoxifen.
The original targeted cancer drug (1977): a pill that blocks oestrogen's effect on breast cancer and halves recurrence, still essential for premenopausal women.
Toremifene (Fareston) is a tamoxifen-like tablet for postmenopausal women with hormone-sensitive breast cancer that has spread.
Vepdegestrant is the first PROTAC ever approved (2026): a pill that tags the oestrogen receptor for destruction, for breast cancers with ESR1 mutations.
Ribociclib is a second adjuvant CDK4/6 option, and the only one with data in node-negative stage II disease. Roughly 3 in 100 patients avoid a relapse or death at three years, so the decision depends heavily on individual risk, tolerance of a three-year oral drug, and cost. Whether the benefit persists after treatment ends, as it did with abemaciclib, needs longer follow-up.
Women with hormone-receptor-positive, HER2-negative breast cancer that has spread to lymph nodes and has other high-risk features can now be offered two years of abemaciclib alongside their hormone therapy, with a durable reduction in relapse. It does not apply to node-negative or low-risk disease, and the diarrhoea and cost are real trade-offs to discuss.
For women at raised risk, a 5-year course of tamoxifen offers long-lasting protection against the commonest kind of breast cancer. Uptake is low because of side effects and fear of rare serious harms; low-dose tamoxifen (TAM-01) is now being tested to improve the balance. It has not been shown to save lives.
Instead of blocking a cancer protein, a drug can now remove it entirely, which works even for proteins without a druggable active site and can overcome resistance driven by target overexpression or mutation. Several degraders are in late-stage trials for breast and prostate cancer.
Query for this target: (TITLE:"Estrogen receptor" OR ABSTRACT:"Estrogen receptor" OR TITLE:"ERα" OR ABSTRACT:"ERα" OR TITLE:"ESR1" OR ABSTRACT:"ESR1") AND (cancer OR tumor OR tumour OR oncology OR carcinoma OR lymphoma OR leukemia OR leukaemia OR myeloma OR sarcoma OR melanoma OR glioma). Results are unfiltered search hits about Estrogen receptor (ERα), not a curated reading list.
Shares Agonist and antagonist, The first PROTAC: a chimeric molecule that tags a protein for destruction, Receptor, Chemical carcinogenesis - receptor activation and the tag hormonal.
Shares lidERA, evERA, Low-dose tamoxifen for high-risk women, prescribed by pharmacists and nurses, VERITAC-2.
Shares Matthew P. Goetz, Debu Tripathy, Massimo Cristofanilli, Stephen Johnston.
Shares evERA, EMERALD, ESR1 mutation (ligand-binding domain, usually in ctDNA), EMBER-3.
Shares lidERA, Giredestrant, ER status (oestrogen receptor by IHC), monarchE: two years of abemaciclib after surgery in high-risk, hormone-receptor-positive early breast cancer.
Shares Endocrine therapy and endocrine resistance, Endocrine resistance, Selective oestrogen receptor degrader (SERD), Hormone receptor status (ER / PR).
Shares Matthew P. Goetz, Stephen Johnston, Komal Jhaveri, EMBER-3.
Shares Debu Tripathy, MONALEESA-2, Seock-Ah Im, ER status (oestrogen receptor by IHC).