HER2 is a protein that helps control how breast cells grow. Some breast cancers make far too much of it and are called HER2-positive; cancers that make normal or low amounts are called HER2-negative. The difference matters because HER2-positive cancers can be treated with medicines designed to target HER2, while HER2-negative cancers cannot — so this one laboratory result helps steer the whole treatment conversation.
Key Takeaways
- HER2 is a growth-related protein found on the surface of breast cells; some cancers produce much more of it than normal.
- HER2-positive means the cancer tested strongly positive for HER2, and HER2-targeted treatments can be discussed.
- HER2-negative means the cancer does not overproduce HER2, so those targeted medicines are not useful against it.
- Testing is done on a tumor sample, usually with a staining test first and a gene-based test when the result is unclear.
- HER2 status is only one part of the picture — hormone receptors, stage, and grade all shape the plan your team proposes.
What HER2 Is, in Plain Language
HER2 (short for human epidermal growth factor receptor 2) is a protein that sits on the surface of cells and receives signals telling the cell to grow and divide. All breast cells have some HER2 — it is a normal part of how tissue grows and repairs itself. Problems arise when a cancer cell makes far too many copies of the HER2 gene, and therefore far too much of the protein. The cell then receives a constant stream of “grow” signals. Testing for HER2 is a standard part of sorting out what kind of breast cancer someone has, as our guide to the types of breast cancer compared sets out.
What HER2-Positive Means
A cancer is HER2-positive when laboratory testing shows the cells are making much more HER2 protein than normal, or that the HER2 gene is amplified — present in extra copies. Because the cancer leans so heavily on HER2 signals, it may grow more quickly than some other types. The important flip side is that this dependence is also a target: medicines have been developed that block HER2 or use it as a flag to deliver treatment to the cancer cells. The arrival of those medicines changed HER2-positive breast cancer from a diagnosis doctors worried more about into one with several effective, specific options. Our guide to HER2-positive breast cancer explained explores this type on its own.
What HER2-Negative Means
HER2-negative simply means the cancer does not overproduce HER2. Most breast cancers fall into this group. It does not mean the cancer has no drivers — hormone receptors may be present instead, or the cancer may be triple-negative if estrogen receptors, progesterone receptors, and HER2 are all absent. It also does not mean the cancer is untreatable; it means the HER2-targeted medicines are not the right tools for it, and other treatments take their place.
You may also come across the term “HER2-low.” Laboratories score HER2 staining on a scale, and results at the low end of negative have recently been grouped under this label, because it may matter for certain treatment decisions in advanced cases. If your report mentions it, ask your care team what it means in your specific situation rather than trying to interpret the score alone.
The Two Side by Side
| Feature | HER2-positive | HER2-negative |
|---|---|---|
| HER2 protein level | Much higher than normal, or the HER2 gene is amplified | Normal or low |
| How the result is reported | Strongly positive staining, or a positive gene test | Negative or low staining, or a negative gene test |
| HER2-targeted medicines | Usually discussed as part of the plan | Not useful, so not used |
| Other treatments discussed | Surgery, chemotherapy, radiation, and hormone therapy if hormone receptors are also present | Surgery, chemotherapy, radiation, and hormone therapy if hormone receptors are present |
| Hormone receptors | Can be present or absent | Can be present or absent |
Notice how much the two columns share. Surgery, radiation, and chemotherapy appear in both, because treatment is built from several pieces of information, and HER2 is only one of them. Your own plan depends on your complete results and circumstances.
How HER2 Testing Works
HER2 is tested on a sample of the tumor, usually from the diagnostic biopsy. The first test is often immunohistochemistry (IHC), which stains the sample to show how much HER2 protein is present and produces a score. If the score is clearly negative or clearly positive, that settles the question. If it lands in the middle, the laboratory usually runs a second test — often shortened to ISH or FISH — which counts copies of the HER2 gene itself. The combined results appear in your pathology report alongside the estrogen and progesterone receptor results, and our guide to ER, PR, and HER2 testing explained describes the process step by step.
Testing is sometimes repeated on the tissue removed during surgery, especially if the biopsy sample was small or the first result was borderline. That is not a sign the first test was wrong — a larger sample simply gives the laboratory more to work with, and the surgical result is generally treated as the more complete picture. If your two results ever differ, ask your team which one they are using to plan treatment, and why.
How the Result Shapes Treatment Discussions
For HER2-positive cancer, doctors usually discuss adding HER2-targeted treatment to the plan, often alongside chemotherapy, and sometimes given before surgery to shrink the tumor. These targeted medicines may continue after surgery for a period of time. Our overview of targeted therapy for HER2-positive breast cancer describes how that category of treatment works in general terms.
For HER2-negative cancer, the plan leans on the other results: hormone therapy if the cancer is hormone-receptor-positive, chemotherapy in many cases, and surgery and radiation as appropriate. In advanced cases, newer medicines — including some that use a low level of HER2 as a delivery target — may enter the discussion, which is why the HER2-low label has become more important. Treatment recommendations change as research develops, and current guidance is published by the American Cancer Society (cancer.org) and the National Cancer Institute (cancer.gov). Your care team can explain which options genuinely apply to you and which do not.
One practical point if HER2-targeted treatment is proposed: because some of these medicines can affect the heart, care teams commonly check heart function before treatment starts and at intervals during it. That monitoring is routine and precautionary. Tell your team about any heart condition you already have, so the checks can be planned properly from the start.
Questions to Ask Your Doctor
- What was my HER2 score, and was a second gene-based test needed to confirm it?
- Does my result count as HER2-positive, HER2-negative, or HER2-low — and what does that mean for me?
- Which treatments does this result make relevant, and which does it rule out?
- How do my hormone receptor results and the stage of the cancer fit with the HER2 result?
- If my situation changes later, would retesting ever be useful?
Frequently Asked Questions
Is HER2 inherited?
No. HER2-positive breast cancer is not passed down in families. The extra HER2 activity happens in the cancer cells themselves. Inherited gene changes that can raise breast cancer risk — such as BRCA1 and BRCA2 — are a separate topic, tested with a blood or saliva sample rather than on the tumor.
Is HER2-positive breast cancer more serious than HER2-negative?
HER2-positive cancers can grow faster, but they also have specific targeted treatments that work well against them. HER2-negative is not automatically “milder,” either — stage, grade, and hormone receptors all affect the picture. Your team can explain what your combined results mean for you.
Can HER2 status change?
Occasionally, yes. Results can differ between the biopsy and the surgical specimen, or if cancer returns in a new place. When that happens, doctors may retest, because the current result is the one that should guide current treatment decisions.
What does an equivocal, or borderline, HER2 result mean?
It means the first staining test was neither clearly positive nor clearly negative, so the laboratory runs a gene-based test to settle it. A borderline first result is common and is a normal part of the testing process, not a cause for alarm.
Do men get HER2-positive breast cancer?
Yes. Breast cancer in men is tested for HER2 in the same way, and the result guides treatment discussions in the same way. The testing pathway does not differ by sex.
Related Articles
- Types of Breast Cancer Compared: The Complete Guide
- HER2-Positive Breast Cancer Explained
- Triple-Negative vs HER2-Positive Breast Cancer: How They Differ
- Targeted Therapy for HER2-Positive Breast Cancer
- ER, PR, and HER2 Testing Explained
This article is for general educational purposes only and is not medical, legal, or financial advice. It does not replace professional care. Talk to your doctor or care team about your own situation. If you think you may have a medical emergency, call 911.