The estrogen receptor (ER) and progesterone receptor (PR) are proteins found inside breast cancer cells that respond to the hormones estrogen and progesterone. When a breast cancer test is positive for these receptors, it means the cancer cells are using hormones as fuel to grow. This information is one of the most important results on a breast cancer pathology report because it directly determines whether hormone-blocking therapies, among the most effective treatments available, are likely to work. ER and PR testing is performed on virtually every newly diagnosed invasive breast cancer and on ductal carcinoma in situ (DCIS).
What the test looks for
Breast cancer cells sometimes retain the same hormone-sensing machinery found in normal breast tissue. This machinery includes proteins called the estrogen receptor and the progesterone receptor. When estrogen or progesterone binds to these receptors, it sends a signal into the cell’s nucleus telling it to divide and grow.
ER/PR testing determines whether a patient’s breast cancer cells carry these receptors. A cancer that carries the estrogen receptor is called ER-positive (ER+). A cancer that carries the progesterone receptor is called PR-positive (PR+). A cancer that carries both is called hormone receptor-positive (HR+). A cancer that carries neither is hormone receptor-negative (HR−).
The distinction matters enormously for treatment. Hormone receptor-positive cancers can be targeted with drugs that either block estrogen from reaching the receptor (tamoxifen), deprive the body of estrogen (aromatase inhibitors such as letrozole, anastrozole, or exemestane), or destroy the receptor itself (fulvestrant). These treatments can be used after surgery to reduce the risk of the cancer coming back, and in advanced disease to slow or stop growth. Hormone receptor-negative cancers do not respond to these therapies, so other treatment strategies are used instead.
Why the test is done
ER and PR testing is a standard part of the workup for all newly diagnosed breast cancers. It is done for several reasons:
- To guide treatment decisions. A positive result opens the door to hormone-blocking therapy (also called endocrine therapy or anti-hormone therapy), which is highly effective and generally well-tolerated compared to chemotherapy.
- To help determine prognosis. Hormone receptor-positive breast cancers tend to grow more slowly than hormone receptor-negative ones, and they are often associated with a better long-term outlook, particularly when treated appropriately.
- To classify the cancer. ER/PR status, combined with HER2 testing and grade, is used to place a breast cancer into one of several molecular subtypes. These subtypes guide decisions about chemotherapy, targeted therapy, and further genomic testing.
- To assess recurrence risk over time. In early-stage hormone receptor-positive cancers, additional genomic tests (such as Oncotype DX or Prosigna) may be ordered to further refine the risk of the cancer returning years later. These tests are only relevant when ER/PR is positive.
How the test is performed
ER and PR testing is performed using a technique called immunohistochemistry (IHC). This is a laboratory method that uses specially designed proteins called antibodies to detect a specific target, in this case the estrogen receptor or progesterone receptor, within cells.
The test is performed on a thin slice of breast tissue taken from the biopsy or surgical specimen. The tissue is mounted on a glass slide and treated with antibodies that attach specifically to ER or PR proteins. A color-developing step makes the antibody visible under a microscope: cells that contain the receptor protein stain brown, while cells without it remain blue. A pathologist then examines the slide and assesses how many cells stain positive and how intensely they stain.
ER and PR testing is typically performed on the initial diagnostic biopsy sample. If the biopsy result is negative and cancer later recurs or spreads, testing is often repeated on the new sample, because receptor status can change over time, particularly after treatment.
How results are reported
ER and PR results in breast cancer are reported using the Allred score or as a simple percentage of positive cells combined with a staining intensity assessment. Different laboratories may use one system or both. The most important number to look for is the percentage of tumor cells that stain positive.
The Allred score
The Allred score is a number from 0 to 8 that combines two assessments:
- Proportion score (0–5). Reflects the percentage of cancer cells staining positive: 0 = none; 1 = less than 1%; 2 = 1–10%; 3 = 10–33%; 4 = 33–66%; 5 = more than 66%.
- Intensity score (0–3). Reflects how strongly the cells stain: 0 = none; 1 = weak; 2 = moderate; 3 = strong.
These two scores are added together. A combined score of 0 or 2 is considered negative. A combined score of 3 or higher is considered positive. (A score of 1 is not possible because proportion and intensity scores cannot independently sum to 1.)
Percentage reporting
Many laboratories report ER and PR simply as a percentage: for example, “ER positive, 90% of cells, strong intensity” or “PR positive, 30% of cells, weak intensity.” Current guidelines from the American Society of Clinical Oncology and the College of American Pathologists define a result as positive if at least 1% of tumor cell nuclei stain positive.
The 1% threshold and low-positive results
A result showing 1–10% of cells staining positive is technically classified as ER-positive or PR-positive under current guidelines, but it occupies a borderline zone. Cancers with very low ER or PR expression (1–10%) may not respond as robustly to endocrine therapy as cancers with higher expression. Some guidelines now distinguish between “ER-low positive” (1–10%) and “ER-positive” (more than 10%) to flag this uncertainty. Your oncologist will take the percentage into account when planning treatment.
What each result means
- ER-positive and PR-positive (ER+/PR+). This is the most common result in breast cancer, occurring in approximately 70–80% of cases. It means the cancer cells carry both hormone receptors and are likely to respond to endocrine therapy. Treatment almost always includes a hormone-blocking medication. The cancer is generally considered to have a relatively favorable outlook compared to hormone receptor-negative subtypes, particularly with appropriate treatment.
- ER-positive and PR-negative (ER+/PR−). The cancer carries the estrogen receptor but not the progesterone receptor. This pattern still qualifies for endocrine therapy, though some research suggests that PR-negative cancers may respond slightly less well to certain hormone-blocking drugs than ER+/PR+ cancers. Your oncologist will weigh this alongside other factors.
- ER-negative and PR-positive (ER−/PR+). This is an uncommon result. Isolated PR positivity without ER expression is biologically unusual and sometimes reflects a technical inconsistency or sampling issue. Repeat testing or review by an expert pathologist may be recommended. The clinical significance of true ER−/PR+ status is debated, but some oncologists will still consider endocrine therapy.
- ER-negative and PR-negative (ER−/PR−). The cancer does not carry either hormone receptor and will not respond to endocrine therapy. The treatment approach will depend on HER2 status and other features. If HER2 is also negative, the cancer is classified as triple-negative breast cancer, a subtype that is treated primarily with chemotherapy and, in eligible patients, immunotherapy or PARP inhibitor therapy.
- ER low-positive (1–10%). This borderline result indicates that a small minority of cancer cells express the estrogen receptor. Whether these cancers benefit meaningfully from endocrine therapy is an active area of research. Your oncologist may recommend additional testing (such as a genomic assay) or may proceed with endocrine therapy given its favorable safety profile.
ER/PR status and breast cancer subtypes
Breast cancers are classified into molecular subtypes based on ER, PR, and HER2 status, combined with tumor grade and the proliferation marker Ki-67. The main subtypes are:
- Luminal A (ER+ and/or PR+, HER2−, low Ki-67, low grade). The most common and generally most favorable subtype. Grows slowly and responds well to endocrine therapy. Chemotherapy is often not needed.
- Luminal B (ER+ and/or PR+, HER2− or HER2+, high Ki-67 or high grade). Hormone receptor-positive but with features suggesting more rapid growth. Chemotherapy is more likely to be recommended alongside endocrine therapy.
- HER2-enriched (ER−, PR−, HER2+). Driven by HER2 overexpression rather than hormones. Treated with HER2-targeted drugs. Endocrine therapy is not used.
- Triple-negative (ER−, PR−, HER2−). Does not respond to endocrine therapy or HER2-targeted drugs. Treated primarily with chemotherapy; immunotherapy is added in some cases.
Understanding which subtype your cancer falls into helps explain why your oncologist is recommending a particular treatment plan.
ER/PR testing in ductal carcinoma in situ (DCIS)
ER and PR testing is also routinely performed on DCIS, the non-invasive form of breast cancer, where abnormal cells are confined within the milk ducts and have not yet spread into surrounding tissue. ER-positive DCIS is often treated with tamoxifen or an aromatase inhibitor after surgery to reduce the risk of the DCIS returning or progressing to invasive cancer. The interpretation of ER/PR results in DCIS follows the same principles as in invasive cancer.
ER/PR testing in metastatic and recurrent breast cancer
If breast cancer returns locally, regionally, or spreads to distant sites such as the bones, liver, or lungs (metastasis), a biopsy of the new site is often performed, and ER/PR testing is repeated. This is because receptor status can change between the original cancer and a recurrence: sometimes a cancer that was hormone receptor-positive becomes negative, or vice versa. Repeat testing ensures that treatment decisions are based on the most current cancer biology.
What happens next
ER and PR results are part of a broader set of tests that together shape your treatment plan. After receiving these results, your care team will typically:
- Combine ER/PR with HER2 and grade. Together, these three results determine your cancer’s molecular subtype, which is the foundation of treatment planning.
- Consider genomic testing if appropriate. For early-stage, ER-positive, HER2-negative cancers, tests such as Oncotype DX, Prosigna, or MammaPrint may be ordered to assess the risk of the cancer returning years later and to determine whether chemotherapy is likely to add benefit beyond endocrine therapy alone.
- Recommend endocrine therapy if ER or PR is positive. This will usually begin after surgery and may continue for 5 to 10 years, depending on your individual risk. The specific drug will depend on your menopausal status and other health factors.
- Discuss chemotherapy. Whether chemotherapy is recommended depends on the stage, grade, subtype, and genomic test results. Not all hormone receptor-positive cancers require chemotherapy.
- Plan follow-up testing. Hormone receptor status is typically re-evaluated if the cancer recurs or spreads.
Questions to ask your doctor
- Is my cancer ER-positive, PR-positive, or both, and what percentage of cells tested positive?
- What does my ER/PR result mean for my treatment options?
- Will I need endocrine therapy, and if so, which medication and for how long?
- Do I need chemotherapy in addition to endocrine therapy?
- Am I a candidate for genomic testing (such as Oncotype DX or Prosigna) to help decide about chemotherapy?
- What is my cancer’s molecular subtype, and what does that mean for my prognosis?
- If my cancer comes back, will the ER/PR testing be repeated?
- Are there clinical trials I should know about based on my ER/PR result?
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