Breast Care A-Z · Concept · ER

Oestrogen receptor

also: ER positive, ER negative, ER status, hormone receptor positive, HR positive, estrogen receptor, progesterone receptor, PR

The oestrogen receptor (ER) is a protein on some breast-cancer cells that lets the hormone oestrogen fuel their growth; cancers with it are called ER-positive and respond to hormone (endocrine) therapy. You may have been told your cancer is ER-positive or negative — together with [HER2](/glossary/her2/) and grade, it decides most of the treatment plan beyond surgery.

Quick answers

Is ER-positive breast cancer better or worse than ER-negative?

ER-positive cancers tend to grow more slowly and respond well to hormone therapy, with generally favourable outlooks for early-stage disease. They also have a longer "tail" of late recurrences, which is why long-term hormone therapy matters. ER-negative cancers grow faster but often respond well to chemotherapy. Neither category is universally "better"; the prognosis depends on the full picture, not just the ER status.

Why is the hormone therapy given for so long?

ER-positive cancers can recur many years after the original treatment — sometimes a decade or more later. The 5-year (and increasingly 10-year) hormone therapy course is designed to reduce that long-tail recurrence risk. Stopping early reduces the protective benefit.

Can I switch hormone therapies if the side effects are difficult?

Yes — many patients switch between tamoxifen and an aromatase inhibitor, or between different aromatase inhibitors, to find the one that suits them. The decision is made with the oncology team. The goal is to find a tolerated treatment that is taken consistently, rather than the "perfect" drug that is too difficult to continue.

BREASTORY ENCYCLOPEDIA · PLATE LVII ONCOLOGY · MOLECULAR PATHOLOGY oestrogen receptor ER · ESR1 gene · oestrogen receptor-positive breast cancer a nuclear hormone receptor driving proliferation in hormone-sensitive breast cancers, predictive of response to endocrine therapy and prognosis i FIG 01 Oestrogen Receptor · Cell Signalling Diagram Cancer cell Nucleus i · ER (ESR1) E2 ii · oestrogen iv · Tamoxifen blocks ER iii · ERE (DNA) Gene transcription → Proliferation (Ki67↑) / Survival / Angiogenesis Adipose tissue → Oestrogen v · Aromatase inhibitor (blocks oestrogen synthesis) Callouts: i=ER nuclear receptor (ESR1) ii=oestrogen binding (E2) iii=ERE DNA binding iv=tamoxifen blockade v=aromatase inhibitor IHC Scoring · Allred Score Allred score: proportion + intensity (0–8) ER+ = Allred >=3 or >=1% · ER-low positive = 1–10% (treat per NG101) ER-low positive 1–10%: benefit uncertain · ER− = <1% Score 0: no staining | Score 8: max expression Reported by pathologist on core biopsy or excision Endocrine Therapy Options Pre-menopausal → tamoxifen 5–10 years Post-menopausal → aromatase inhibitor (AI) Both → ovarian suppression + AI (high-risk pre-meno) letrozole / anastrozole / exemestane GnRH agonist (goserelin) for ovarian suppression ii FIG 02 ER-positive vs ER-negative Breast Cancer Feature ER-positive (~75%) ER-negative (~25%) Molecular subtype Luminal A / B HER2-enriched or TNBC Prognosis Better overall Worse short-term Grade Often 1–2 Often grade 3 Ki67 Lower Higher Hormone therapy Yes (tamoxifen / AI) Not effective Chemotherapy Selected cases Usually required Late recurrence Yes (>5 years) Less common ESR1 mutation Acquired resistance Not applicable 5-yr survival ~90%+ ~80% iii FIG 03 Endocrine Therapy Drug Reference Drug Mechanism Use Tamoxifen Competitive ER blocker Pre- and post-menopausal Letrozole Aromatase inhibitor Post-menopausal Anastrozole Aromatase inhibitor Post-menopausal Exemestane Steroidal AI (irreversible) Post-menopausal Goserelin GnRH agonist (ovarian suppression) Pre-menopausal Fulvestrant ER degrader (SERD) Advanced / metastatic Elacestrant Oral SERD ESR1-mutant advanced disease CDK4/6 inhibitors (metastatic only; abemaciclib adjuvant TA852) Cell cycle arrest (+ AI) HR+ advanced Abemaciclib (NICE TA852 — adjuvant) CDK4/6 inhibitor Adjuvant high-risk iv FIG 04 Clinical Pathway · ER Testing to Endocrine Therapy 1 Core biopsy 2 ER IHC testing (Allred score) 3 ER status reported 4 Oncotype DX genomic testing 5 Adjuvant endocrine Rx 6 5–10 yr treatment Genomic testing (Oncotype DX / Prosigna) guides chemotherapy decision in intermediate-risk ER+ cases v FIG 05 Key Concepts & Drug Reference ER-positive (Luminal A) Low-grade, good prognosis ER-positive (Luminal B) Higher grade, Ki67 elevated Tamoxifen Pre-menopausal first-line Aromatase inhibitor (AI) Post-menopausal standard Ovarian suppression + AI High-risk pre-menopausal ESR1 mutation Acquired resistance mechanism vi FIG 06 Key Statistics · ER-positive Disease ~75% of breast cancers are ER-positive [1] ~40% reduction in recurrence (tamoxifen) [2] Superior AI vs tamoxifen post-menopause [3] +13 mo PFS with CDK4/6 inhibitor (metastatic) [4] vii FIG 07 References 1. NICE NG101. Early breast cancer 2023 2. EBCTCG. Tamoxifen meta-analysis. Lancet 2011;378:771 3. EBCTCG. AI meta-analysis. Lancet 2015;386:1341 4. Finn RS et al. PALOMA-2 trial. NEJM 2016;375:1925 5. WHO Classification of Tumours: Breast 5th ed. IARC 2022 Clinically authored by Dr Fiona Tsang-Wright , FRCS (Gen Surg) GMC 4549831 · ORCID 0000-0003-4801-026X
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Definition
The oestrogen receptor (ER) is a protein in some breast cells that binds to the hormone oestrogen and signals the cell to grow — and breast cancers that test "ER-positive" can be effectively treated with hormone-blocking therapies that interfere with this signal.

Common questions The questions patients ask first

Is ER-positive breast cancer better or worse than ER-negative?
ER-positive cancers tend to grow more slowly and respond well to hormone therapy1, with generally favourable outlooks for early-stage disease. They also have a longer 'tail' of late recurrences, which is why long-term hormone therapy matters4. ER-negative cancers grow faster but often respond well to chemotherapy. Neither category is universally 'better'; the prognosis depends on the full picture, not just the ER status.
Why is the hormone therapy given for so long?
ER-positive cancers can recur many years after the original treatment — sometimes a decade or more later. The 5-year (and increasingly 10-year) hormone therapy course is designed to reduce that long-tail recurrence risk4. Stopping early reduces the protective benefit.
Can I switch hormone therapies if the side effects are difficult?
Yes — many patients switch between tamoxifen and an aromatase inhibitor, or between different aromatase inhibitors, to find the one that suits them3. The decision is made with the oncology team. The goal is to find a tolerated treatment that is taken consistently, rather than the 'perfect' drug that is too difficult to continue.
Will hormone therapy stop my periods?
In pre-menopausal patients, tamoxifen alone usually does not stop periods, though it can make them irregular. Ovarian suppression drugs do stop periods (and induce a temporary menopause). Aromatase inhibitors are usually only used in post-menopausal patients or alongside ovarian suppression3.

The oestrogen receptor (ER) is a protein in some breast cells that binds to the hormone oestrogen and signals the cell to grow — and breast cancers that test “ER-positive” can be effectively treated with hormone-blocking therapies that interfere with this signal.

Around 70–80% of breast cancers are oestrogen-receptor positive2. ER status is tested on the biopsy or surgical specimen and reported as positive or negative. A second receptor — progesterone receptor (PR) — is tested alongside. ER-positive cancers respond to hormone therapy (tamoxifen, aromatase inhibitors), typically given for 5–10 years after surgery.

Orientation Why you might be reading about this

You may have been told your cancer is ER-positive (or negative), or you are reading to understand what receptor status means. Together with HER2 and grade, ER is one of the three biological features that decide most of the treatment plan beyond surgery itself. This page explains what the oestrogen receptor is, what positive and negative results mean, and what hormone therapy involves.

Related terms: HER2 · Invasive ductal carcinoma · Invasive lobular carcinoma · Core biopsy · Grade and stage

Mechanism How the oestrogen receptor drives cancer growth

Oestrogen is a hormone that plays a normal role in breast development and in the menstrual cycle. It works by binding to oestrogen receptors inside breast cells, which then activate growth pathways. In normal breast tissue, this is a regulated, time-limited signal.

In some breast cancers, the cancer cells retain the oestrogen receptor and continue to use oestrogen as a growth signal. These cancers are called oestrogen-receptor positive (ER-positive). The clinical importance is twofold:

  1. They respond to hormone therapy. Drugs that block the oestrogen signal — by blocking the receptor itself, by stopping oestrogen production, or by destroying the receptor — effectively starve the cancer of its growth signal.
  2. They tend to grow more slowly than ER-negative cancers, with later patterns of recurrence — which is why hormone therapy is given for years after surgery, not just a few months.

The companion test, progesterone receptor (PR), is usually done at the same time. PR-positive cancers are similar to ER-positive cancers; the combination ER+/PR+ is more reliably hormone-responsive than ER+/PR– alone.

Testing How ER is tested

ER status is tested on the biopsy or operation specimen using immunohistochemistry — a staining test that detects the receptor inside cell nuclei. The result is reported as a percentage of cells staining positive, and as a category:

  • ER-positive — typically 1% or more of cells stain positive5. Most pathology reports give the actual percentage.
  • ER-negative — fewer than 1% of cells stain positive5.

The same approach is used for PR.

A small group of cancers test “ER-low” — between 1% and 10% positive — and behave somewhere between ER-positive and ER-negative5. These cases are individually discussed at MDT3.

ER-positive Treatment for ER-positive cancer

Hormone therapy is the cornerstone of treatment for ER-positive cancer, alongside surgery and (in some cases) chemotherapy and targeted therapy.

Pre-menopausal patients

  • Tamoxifen — the standard treatment, taken as a daily tablet for 5 to 10 years1. Works by blocking the oestrogen receptor itself.
  • Ovarian function suppression — drugs (or surgery, occasionally) that switch off oestrogen production from the ovaries. Sometimes added to tamoxifen, or used to allow an aromatase inhibitor to work in younger patients.

Post-menopausal patients

  • Aromatase inhibitors — anastrozole, letrozole, or exemestane1. Daily tablets, typically for 5 to 10 years. Work by stopping the small amount of oestrogen the body still produces after the menopause.
  • Tamoxifen is sometimes used instead, where aromatase inhibitors are not tolerated.

Hormone therapy duration is 5 years as standard, with extension to 10 years146 for higher-risk cases or where it is being well tolerated. The choice and duration are individualised3.

ER-negative Treatment for ER-negative cancer

ER-negative cancers do not respond to hormone therapy and treatment is built around chemotherapy (where appropriate), HER2-targeted therapy (if HER2-positive), and — for the triple-negative subgroup (ER-, PR-, HER2-) — chemotherapy plus selected newer agents (immunotherapy, PARP inhibitors in BRCA carriers).

ER-negative cancers tend to grow faster than ER-positive cancers, but they also tend to respond more dramatically to chemotherapy. Recurrence patterns are different — earlier peaks for ER-negative; later, more delayed recurrences for ER-positive.

Side effects Side effects of hormone therapy

Hormone therapy is taken for years, so its side effects are an important part of the conversation. Common ones include:

  • Hot flushes and other menopausal symptoms.
  • Joint aches (more common with aromatase inhibitors).
  • Mood and sleep changes.
  • Reduced libido and vaginal dryness.
  • Bone-density loss with aromatase inhibitors — monitored with periodic DEXA scans, with calcium / vitamin D supplementation or bone-protecting medication when needed.
  • Small increased risk of thrombosis with tamoxifen.

Most patients tolerate hormone therapy well, with manageable side effects. Patients who find side effects intolerable can usually switch between drugs (tamoxifen ↔ aromatase inhibitor) — the side-effect profiles differ, and most patients tolerate at least one of the options.

At consultation What to discuss at consultation

Once ER status is known:

  • What hormone therapy is being recommended and why — drug, duration, expected benefit.
  • Side effects and how they will be managed.
  • Bone health monitoring if an aromatase inhibitor is being used.
  • The interaction with chemotherapy decisions — modern genomic tests (Oncotype DX, MammaPrint) help work out whether ER-positive cancers benefit from chemotherapy on top of hormone therapy7.

Resources Further reading

Sources & guidance

Every figure on this page is anchored to a published source. Tap a number in the text or below to jump to the reference.

  1. meta analysis Early Breast Cancer Trialists' Collaborative Group (EBCTCG). Relevance of breast cancer hormone receptors and other factors to the efficacy of adjuvant tamoxifen: patient-level meta-analysis of randomised trials. The Lancet. 2011 ;378(9793):771-784 doi:10.1016/S0140-6736(11)60993-8 Cited for: Magnitude and durability of tamoxifen benefit in ER-positive breast cancer; ~50% reduction in recurrence over 5 years.
  2. cohort Howlader N, Altekruse SF, Li CI, et al. US incidence of breast cancer subtypes defined by joint hormone receptor and HER2 status. Journal of the National Cancer Institute. 2014 ;106(5):dju055 doi:10.1093/jnci/dju055 Cited for: Population-level breast cancer subtype frequencies — ER-positive cancers account for ~70% of breast cancers.
  3. guideline National Institute for Health and Care Excellence (NICE). Early and locally advanced breast cancer: diagnosis and management (NG101). London: NICE. 2018 ;Last updated 2024 https://www.nice.org.uk/guidance/ng101 Cited for: UK pathway for endocrine therapy in early breast cancer; ER positivity threshold; choice between tamoxifen and aromatase inhibitor by menopausal status.
  4. rct Davies C, Pan H, Godwin J, et al. (ATLAS Collaborative Group). Long-term effects of continuing adjuvant tamoxifen to 10 years versus stopping at 5 years after diagnosis of oestrogen receptor-positive breast cancer: ATLAS, a randomised trial. The Lancet. 2013 ;381(9869):805–816 doi:10.1016/S0140-6736(12)61963-1 Cited for: Extending tamoxifen from 5 to 10 years further reduces recurrence and breast-cancer mortality — rationale for 5–10-year endocrine therapy duration.
  5. guideline Allison KH, Hammond MEH, Dowsett M, et al. (American Society of Clinical Oncology / College of American Pathologists). Estrogen and progesterone receptor testing in breast cancer: ASCO/CAP guideline update. Journal of Clinical Oncology. 2020 ;38(12):1346–1366 doi:10.1200/JCO.19.02309 Cited for: Threshold for ER-positive (≥1% nuclear staining); 'ER-low' category (1–10%); standardised IHC reporting.
  6. rct Goss PE, Ingle JN, Pritchard KI, et al. Extending Aromatase-Inhibitor Adjuvant Therapy to 10 Years. New England Journal of Medicine. 2016 ;375(3):209-219 doi:10.1056/NEJMoa1604700 Cited for: MA.17R: extending aromatase inhibitor therapy to 10 years reduces recurrence in postmenopausal ER-positive breast cancer.
  7. rct Sparano JA, Gray RJ, Makower DF, et al. Adjuvant chemotherapy guided by a 21-gene expression assay in breast cancer. New England Journal of Medicine. 2018 ;379(2):111-121 doi:10.1056/NEJMoa1804710 Cited for: TAILORx: 21-gene Oncotype DX recurrence-score-guided chemotherapy decisions in ER-positive HER2-negative early breast cancer.