The short answer
ER and PR status tell whether breast cancer cells have receptors for the hormones estrogen and progesterone. Positive results mean hormone therapy may help.
ER means estrogen receptor; PR means progesterone receptor.
These receptors are found on some breast cancer cells.
'Positive' means the cells have the receptors and may respond to hormone therapy.
'Negative' means the cells lack them, so hormone therapy is less likely to help.
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The full explanation.
What the letters mean
ER stands for estrogen receptor. PR stands for progesterone receptor. Both are proteins found on some breast cancer cells. Receptors act like docking stations. When the hormones estrogen or progesterone attach to these stations, they can signal the cell to grow.
A pathologist tests your breast cancer tissue for both receptors. The test uses a stain called immunohistochemistry, or IHC. The result tells your care team whether these hormones are likely fueling your cancer.
Reading positive and negative, including "low positive"
A positive result means the cancer cells carry these receptors. This is reported as a percentage, from 0% to 100% of cells staining. Staining in 10% or more of cells is usually called clearly positive. A negative result means fewer than 1% of cells stain. The cancer has essentially none of these receptors.
Between those two, there is a middle category. Current guidelines from ASCO and the College of American Pathologists call it "ER-low positive." It covers staining in roughly 1% to 10% of cells. This group can behave somewhat differently than clearly positive or clearly negative cancers. Your oncologist may discuss it specifically if it applies to you.
Why it guides treatment
ER and PR status is one of the most useful parts of a breast cancer report. It points directly at whether hormone therapy is likely to help. Hormone therapy works by blocking estrogen, lowering estrogen levels, or blocking the receptor itself. If cells carry the receptor, this approach can meaningfully lower the chance the cancer returns. If they do not, hormone therapy is not expected to work. There is no receptor for the drug to act on.
Most breast cancers are hormone-receptor positive. That is one reason hormone therapy is such a common part of treatment.
The bigger biomarker panel
ER and PR are almost always tested together with a third marker, HER2. When all three come back negative, the cancer is called triple-negative. It is treated with different approaches, mainly chemotherapy, and in some cases immunotherapy. Your pathology report should list ER, PR, and HER2 together. They are read as one panel, not separately.
What it does not tell you
Receptor status does not tell you the stage of your cancer. That means its size, and whether it has spread. It also does not guarantee how well hormone therapy will work for you. It identifies who is a reasonable candidate, not a guaranteed responder. Receptor status is measured from the tissue tested. In rare cases, especially if cancer returns years later, a repeat biopsy shows a different result than the original tumor. Cancer biology can shift over time.
Is this urgent?
ER and PR testing is routine and expected. It is not an emergency finding on its own. What matters is making sure results are ready before treatment planning begins, since they shape which drugs are offered.
What to ask your team
- What were my exact ER and PR percentages, not just positive or negative?
- Am I in the "ER-low positive" category, and does that change my options?
- What does this mean for hormone therapy specifically?
- How do these results fit with my HER2 status?
How the test is actually scored
A pathologist looks at your tumor sample under a microscope after IHC staining. They count what fraction of the cancer cell nuclei show brown staining for each receptor. Some labs also note how strong the staining looks, not just how many cells stain. Exact percentages can vary a little between labs, which is why the ASCO/CAP guideline asks laboratories to have a set procedure for confirming results when staining is low, which is one reason your care team looks at the overall pattern rather than treating a single percentage point as an exact cutoff.
Why testing happens more than once for some people
ER and PR are usually tested once, on your original tumor sample. But if your cancer returns years later, doctors sometimes repeat the test on the new tissue, since receptor status can change between the original tumor and a recurrence. A change from positive to negative, or the reverse, is uncommon but does happen, and when it does, it can shift which treatments make sense. If you are facing a recurrence, ask whether repeat receptor testing is being done on the new tissue rather than relying on your original result alone.
Sources
Words to know
Tap any term to see what it means.
Common questions
What are ER and PR?
They are receptors — docking points — on some breast cancer cells for the hormones estrogen (ER) and progesterone (PR). Their presence affects how the cancer may be treated.
What does ER-positive mean?
It means the cancer cells have estrogen receptors, so estrogen can encourage them to grow. These cancers often respond to hormone therapy that blocks or lowers estrogen.
Is ER-positive or ER-negative better?
ER-positive cancers can be treated with hormone therapy, which is an advantage. Outlook depends on many factors, so no single result defines it.
How is this related to HER2 and triple-negative?
ER, PR, and HER2 are tested together. When all three are negative, the cancer is called triple-negative, which is treated differently.
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Written by: Cancer ExplainedSources last checked: 2026-07-14 what this meansLast updated: 2026-08-18Next planned review: 2027-07-14
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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