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.
Where you'll see this phrase
On the pathology or biomarker report for breast cancer, and some stomach and esophageal cancers. You may see "HER2 (IHC): 2+," "HER2 negative," "HER2 positive (FISH amplified)," or "HER2 low."
What it means in plain language
HER2 is a protein on the surface of cells. Some cancers make too much of it, which can drive faster growth. Testing measures how much HER2 is present:
- HER2 positive — high levels; HER2-targeted medicines may be effective.
- HER2 negative — normal/low levels by the traditional cutoff.
- HER2 low — some HER2, below the "positive" line, but now meaningful because newer targeted drugs can work here.
Two tests are common: IHC (scores staining from 0 to 3+) and FISH (counts HER2 gene copies). Teams sometimes use both.
Why it matters
HER2 status is one of the main results that steer treatment choice. HER2-targeted therapies have transformed options for HER2-positive cancers, and the newer HER2-low category has expanded who may benefit from certain targeted drugs.
What it does not mean
- HER2 status does not tell you the stage — how far the cancer has spread.
- A single result is not the whole picture; hormone receptor status and other markers matter too.
What context is still needed
HER2 testing has specific cutoffs and occasional retesting when results are borderline. Your oncologist interprets your exact result alongside the rest of your report.
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.
Questions to ask your doctor
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Your next step
Plain-language definitions for the words on your report.
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