The short answer
FISH stands for fluorescence in situ hybridization. Fluorescent DNA probes stick to chosen genes or chromosome regions and glow under a microscope. It answers only the question its probes were built for, so the result depends entirely on what was ordered.
FISH is a method, so seeing the word tells you how something was tested, not what was found.
NCI says it can show where a gene sits on a chromosome, how many copies are present, and chromosome abnormalities.
Probes only light up their own targets, so FISH is blind to everything nobody asked about.
In HER2 breast cancer testing, the ASCO and CAP guideline requires FISH-style counts and the stain to be read together.
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The full explanation.
A method, not a finding
FISH stands for fluorescence in situ hybridization. Seeing it on your report tells you how the lab looked at your sample. It does not tell you what they saw.
NCI describes the technique in three steps. Pieces of DNA carrying a fluorescent dye are made in the lab. They are added to a cell or tissue sample. Where they bind to their matching gene or chromosome region, they glow under a microscope with a special light.
So the report has two halves. The method, and the result. Read the second half.
What the glowing dots are counting
Each dot is one place a probe found its match. A pathologist counts dots in a set number of cells and compares the totals.
NCI says FISH can be used to identify where a specific gene sits on a chromosome, how many copies of the gene are present, and any chromosome abnormalities. Those three uses map onto the three answers you are likely to see:
- extra copies, reported as amplification.
- missing copies, reported as deletion or loss.
- pieces of two chromosomes joined together, reported as a rearrangement or fusion.
The Philadelphia chromosome is the classic example of that last one. NCI describes it as part of the ABL1 gene on chromosome 9 breaking off and attaching to the BCR gene on chromosome 22.
Where it gets ordered
FISH appears wherever a specific, known change decides something.
It shows up in breast cancer reports counting HER2 copies. It shows up in leukemia and lymphoma work, where fusions define subtypes. It shows up in sarcoma, where a single rearrangement can settle which tumor type you are dealing with. Bone marrow reports often list it alongside a full chromosome count.
In each case, someone chose the probe before the test ran.
What FISH deliberately does not see
This is the limitation worth understanding, because it explains why more tests follow.
A probe binds only where its own sequence matches. A HER2 probe says nothing about any other gene. FISH also reads copy number and position, not the letter-by-letter sequence, so it will not find a small point mutation.
A normal FISH result therefore means one thing: the specific change that probe was looking for was not found. It is not a clean bill of genetic health.
How it fits with the other tests
Guidelines often pair FISH with a stain rather than choosing between them.
In HER2 breast cancer testing, the ASCO and College of American Pathologists guideline sets out a dual-probe approach that compares HER2 copies with a chromosome 17 reference. For the less common counting patterns, it requires the stain to be reviewed alongside the counts before a final positive or negative is assigned.
Broader sequencing panels usually run separately and report later. NCI also lists cytogenetic analysis, which counts whole chromosomes and looks for broken, missing, rearranged or extra ones.
What Does HER2 Status Mean? covers the breast cancer setting. Biomarker Testing and Precision Medicine explains how these results feed into treatment choices.
Sources
- NCI Dictionary of Cancer Terms — FISH
- NCI Dictionary of Cancer Terms — gene amplification
- NCI Dictionary of Cancer Terms — Philadelphia chromosome
- Wolff AC et al. HER2 Testing in Breast Cancer: ASCO/CAP Clinical Practice Guideline Focused Update. Arch Pathol Lab Med. 2018;142(11):1364-1382
- National Cancer Institute — Tests and Procedures Used to Diagnose Cancer
Words to know
Tap any term to see what it means.
Common questions
Does a FISH test mean I have cancer?
No. FISH is a laboratory technique, not a diagnosis. NCI describes it as a way to look at genes or chromosomes in cells and tissues, used to help diagnose diseases such as cancer and to help plan treatment. What matters is which probe was used and what it showed.
How does it actually work?
NCI describes it plainly. Pieces of DNA carrying a fluorescent dye are made in the lab and added to a cell or tissue sample. Where those pieces bind to their matching gene or chromosome region, they light up under a microscope with a special light.
Why did my doctor order FISH when I already had sequencing?
They answer different questions. FISH is good at counting copies of a gene and at seeing pieces of chromosomes that have swapped places. Some of those rearrangements are hard to call from sequencing alone.
What does amplified mean on a FISH report?
NCI defines gene amplification as an increase in the number of copies of a gene. It is common in cancer cells, and some amplified genes drive growth or cause resistance to anticancer drugs.
Questions to ask your doctor
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Sources last checked: 2026-08-09 what this meansLast updated: 2026-08-17Next planned review: 2027-08-09
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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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