biomarkersBiomarker

ER status (oestrogen receptor by IHC)

aka ER status, ER-positive, ER positive, ER+, ER-negative, estrogen receptor status, oestrogen receptor status, ER low positive, ER >= 1%, ER 1-10%

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.

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.

Part ofEstrogen receptor (ERα)· ESR1Other readoutsESR1 mutation All readouts

Estrogen receptor (ERα): what it is and how drugs act on it · animated schematic, not to scale

Target · the protein and the cell it sits on
Drug · antibody, small molecule, cell or radioligand
Effect · signal, damage or kill

In plain words · The hormone switch that drives most breast cancers. Blocking or destroying it is the oldest and most effective targeted therapy.

IHC score 3 approval thresholds

What it means for a patient

If 1 percent or more of your tumour cells stain for the oestrogen receptor, the cancer is ER-positive and hormone-blocking drugs (tamoxifen, aromatase inhibitors, fulvestrant) are part of treatment, usually with a CDK4/6 inhibitor when the cancer has spread. A result between 1 and 10 percent is 'low positive'; your team may treat it more like ER-negative disease and discuss chemotherapy.

Written only from the label or guideline text cited on this page. Not medical advice; your own report and the reading your team gives it come first.

Scoring rule

ER-positive: at least 1 percent of tumour cell nuclei stain positive by IHC. ER low positive: 1 to 10 percent. ER-negative: less than 1 percent or no staining, with internal and external controls behaving as expected.

“ER-positive... if ≥ 1% of tumor cell nuclei are immunoreactive”
ASCO/CAP estrogen and progesterone receptor testing guideline, 2020 update (Allison et al., J Clin Oncol)

Thresholds used in approvals

Threshold	Drug	Cancer	Regulator	Source
ER-positive (guideline >= 1% nuclei)	Elacestrant	HR-positive / HER2-negative breast cancer	FDA	label
Hormone receptor-positive (guideline >= 1% nuclei)	Alpelisib	HR-positive / HER2-negative breast cancer	FDA	label
Hormone receptor-positive (guideline >= 1% nuclei)	Capivasertib	HR-positive / HER2-negative breast cancer	FDA	label

Also defined by ASCO/CAP estrogen and progesterone receptor testing guideline, 2020 update (Allison et al., J Clin Oncol).

Drugs whose label names it

Elacestrant Alpelisib Capivasertib

Tests that measure it

Exact Sciences Oncotype DX Breast Recurrence Score

Trials that name this readout

Matched on the name and aliases of the readout in the title, setting and summary of each trial; a match is a mention, not proof the readout was an entry criterion.