# ER status (oestrogen receptor by IHC)

Source: https://onco.cc/biomarkers/er-status/  
OnCo record `er-status` (Biomarker). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

ER status is whether the tumour's cells carry the oestrogen receptor, read by staining nuclei. One percent or more of stained nuclei is positive and means hormone-blocking treatment can work; 1 to 10 percent is 'low positive' and behaves more like negative.

## Summary

The ASCO/CAP 2020 guideline calls a breast cancer ER-positive when at least 1 percent of tumour cell nuclei stain by validated immunohistochemistry, ER-negative below 1 percent, and introduces 'ER low positive' for 1 to 10 percent with a comment that these tumours often behave like ER-negative disease. Every endocrine therapy label (tamoxifen, aromatase inhibitors, fulvestrant) and the CDK4/6 inhibitor, PI3K, AKT and oral SERD labels are written for hormone-receptor-positive or ER-positive disease without restating the 1 percent rule, so the guideline threshold is what laboratories apply. The same 1 percent rule is used for the progesterone receptor.

## Fields

- Kind: Biomarker
- Last checked: 2026-09-23
- Also known as: ER status; ER-positive; ER positive; ER+; ER-negative; estrogen receptor status; oestrogen receptor status; ER low positive; ER >= 1%; ER 1-10%
- Tags: biomarker; hormone-receptor

## Sources

- ASCO/CAP estrogen and progesterone receptor testing guideline, 2020 update (Allison et al., J Clin Oncol): https://doi.org/10.1200/JCO.19.02309

## Connected records

- biomarkers: [ESR1 mutation (ligand-binding domain, usually in ctDNA)](https://onco.cc/biomarkers/esr1-mutation-ctdna/), [Ki-67 index (proliferation by IHC)](https://onco.cc/biomarkers/ki-67-index/), [PR status (progesterone receptor by IHC)](https://onco.cc/biomarkers/pr-status/)
- cancers: [Breast cancer (all types)](https://onco.cc/cancers/breast-cancer/), [Endometrial cancer](https://onco.cc/cancers/endometrial/), [High-risk early HR-positive breast cancer](https://onco.cc/cancers/hr-positive-early-high-risk/), [HR-positive / HER2-negative breast cancer](https://onco.cc/cancers/breast-hr-positive/), [HR-positive metastatic breast cancer after CDK4/6 inhibitors](https://onco.cc/cancers/hr-positive-metastatic-post-cdk46/)
- drugs: [Abemaciclib](https://onco.cc/drugs/abemaciclib/), [Alpelisib](https://onco.cc/drugs/alpelisib/), [Capivasertib](https://onco.cc/drugs/capivasertib/), [Elacestrant](https://onco.cc/drugs/elacestrant/), [Fulvestrant](https://onco.cc/drugs/fulvestrant/), [Letrozole (and other aromatase inhibitors)](https://onco.cc/drugs/letrozole/), [Palbociclib](https://onco.cc/drugs/palbociclib/), [Ribociclib](https://onco.cc/drugs/ribociclib/), [Tamoxifen](https://onco.cc/drugs/tamoxifen/)
- terms: [Hormone receptor status (ER / PR)](https://onco.cc/terms/hormone-receptor-status/), [Immunohistochemistry (IHC)](https://onco.cc/terms/ihc/), [Selective oestrogen receptor degrader (SERD)](https://onco.cc/terms/serd/)
- targets: [Estrogen receptor (ERα)](https://onco.cc/targets/estrogen-receptor/)

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JSON: https://onco.cc/api/v1/entities/er-status.json