The short answer
HER2 is a protein that can appear in large amounts on some cancer cells and help them grow. Testing sorts tumors into HER2 positive and HER2 negative, with some results called equivocal until a second test settles them. The result helps your team determine treatment; it does not describe how far the cancer has spread.
HER2 is a protein tested in breast cancer and several other cancers.
HER2 positive means a strong test result (IHC 3+, or a positive ISH test); HER2 status helps determine treatment.
HER2 status guides treatment choice — it is not the cancer's stage or grade.
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This page is for people with breast cancer, or another cancer tested for HER2, who want to understand their HER2 result. It covers how HER2 is tested, what each score means, and why it matters for treatment.
What HER2 is
HER2 is a protein that sits on the surface of cells and helps control how they grow and divide. HER2 is tested in breast cancer and several other cancers, including ovarian, bladder, pancreatic, non-small cell lung, and stomach or gastroesophageal cancers. The National Cancer Institute lists the HER2 result as helping to determine treatment.
How HER2 is tested
In breast cancer, the NCI clinician summary (PDQ) describes two kinds of tests. Immunohistochemistry (IHC) scores staining from 0 to 3+. In situ hybridization (ISH, which includes FISH) looks at HER2 gene copies.
- IHC 0 or 1+ is negative.
- IHC 2+ is equivocal, meaning unclear on its own. It needs ISH testing.
- IHC 3+ is positive.
ISH results are also reported as negative, equivocal, or positive, based on the number of HER2 gene copies and a ratio of HER2 to a control gene.
Why the category matters for treatment
HER2 status helps determine which treatments are used. Your team decides which treatments fit your cancer. Ask what your result means for your options.
What your result does not tell you
Your HER2 result does not, by itself, determine your full treatment plan. It works alongside your hormone receptor status, your cancer's stage, and other features. If your cancer returns, ask whether testing will be repeated.
What to ask your team
- What is my exact HER2 category, and was FISH testing needed to confirm it?
- Does my HER2 status open up specific targeted therapy options?
- Would my HER2 status be retested if my cancer changes or returns?
Sources
Common questions
Is HER2 positive worse than HER2 negative?
It is not simply better or worse. HER2 status helps your team determine treatment. Your team weighs it with everything else about your cancer.
What if my report says HER2 low?
Some reports use extra labels for results in the negative or equivocal range. Ask your care team what your exact result means and whether it changes your treatment options.
Questions to ask your doctor
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Last updated: 2026-10-16Next planned review: 2027-07-12
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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, and this 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 then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. 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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