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, HER2 negative, and — more recently — HER2 low. The result matters because HER2-targeted medicines work against HER2-positive (and some HER2-low) cancers. Your HER2 status helps your team choose treatments; it does not describe how far the cancer has spread.
HER2 is a growth-related protein tested mainly in breast and some stomach/esophageal cancers.
HER2 positive means high HER2 levels; targeted HER2 medicines may be options.
HER2 low is a newer category that can also open certain targeted treatment options.
HER2 status guides treatment choice — it is not the cancer's stage or grade.
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The full explanation.
What HER2 is
HER2 is a protein that sits on the surface of cells and helps control how they grow and divide. Every cell has some HER2. In about 15% to 20% of breast cancers, and in some other cancers too, the tumor cells make far too much of it. That extra HER2 pushes cells to grow and divide faster than they should. Testing your tumor for HER2 tells your team whether this particular growth signal is driving your cancer, which determines whether HER2-targeted drugs will help.
How HER2 is tested
The first test is usually immunohistochemistry, or IHC, which uses a chemical stain to show how much HER2 protein sits on the surface of your cancer cells. A pathologist scores the result from 0 to 3+, based on how strongly and completely the cells stain.
- 0 means no meaningful HER2 staining. This is HER2-negative, sometimes called HER2-zero.
- 1+ means faint, incomplete staining. This is called HER2-low.
- 2+ is called equivocal, meaning the result is unclear on its own. It needs a second, more precise test called FISH to settle the question, by counting the actual number of HER2 gene copies in the cells.
- 3+ means strong, complete staining. This is HER2-positive.
If FISH confirms extra gene copies in a 2+ tumor, that tumor is also classified as HER2-positive. If FISH does not confirm extra copies, the tumor falls into HER2-low instead.
Why the category matters for treatment
HER2-positive tumors, meaning IHC 3+ or FISH-confirmed 2+, are treated with HER2-targeted drugs like trastuzumab and pertuzumab, often combined with chemotherapy. These drugs specifically block the extra HER2 protein driving the cancer's growth, and they have substantially improved outcomes for HER2-positive breast cancer since they came into use.
For a long time, HER2-low tumors, meaning IHC 1+ or 2+ without FISH confirmation, were treated as though they had no meaningful HER2 signal at all, grouped in with HER2-negative cancers. That changed with the development of a newer drug called trastuzumab deruxtecan, which was shown to work even against these lower levels of HER2, opening up a treatment option that did not exist for this group before.
What your result does not tell you
Your HER2 category does not, by itself, determine your full treatment plan. It works alongside your hormone receptor status, your cancer's stage, and other features to shape the overall approach. It also is not necessarily permanent — HER2 status can occasionally differ between the original tumor and a later recurrence, which is part of why retesting sometimes happens if your cancer returns or spreads.
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?
- If I am HER2-low, does that change my treatment plan compared to HER2-zero?
- Would my HER2 status be retested if my cancer changes or returns?
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Words to know
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Common questions
Is HER2 positive worse than HER2 negative?
It's not simply better or worse. HER2-positive cancers can grow faster, but there are effective HER2-targeted treatments, which has greatly improved options. Your team weighs HER2 with everything else.
What is HER2 low?
HER2 low means the cancer has some HER2 but not enough to be called positive by the older cutoff. Newer targeted drugs can work in some HER2-low cancers, so this category now affects treatment planning.
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Last updated: 2026-08-18Next planned review: 2027-07-12
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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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