The short answer
Microsatellite instability (MSI) is a sign that a tumor's DNA repair system is not working well. MSI-high tumors may respond to immunotherapy and can point to an inherited syndrome.
MSI shows whether a tumor can properly repair certain DNA errors.
'MSI-high' means many repair errors have built up.
MSI-high tumors may respond well to immunotherapy.
MSI-high can be a clue to Lynch syndrome, an inherited condition.
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This page is for people who see "microsatellite instability" or "MSI" on a tumor test report. It explains the result, how it can relate to immunotherapy, and what it may mean for family.
What the term describes
Microsatellites are short, repeating stretches of DNA. Microsatellite instability, or MSI, means these repeats show abnormal patterns in the tumor. It is linked to a problem with the cell's mismatch repair system, which fixes small DNA copying errors.
MSI-high and what it signals
A tumor with many such changes is called MSI-high. This can matter in two ways. First, it can open a treatment path with immune checkpoint inhibitors. NCI notes these drugs can be used for any solid tumor that cannot repair errors in its DNA. Second, it can be a clue about Lynch syndrome, an inherited condition. NCI genetics summaries say abnormal MSI or mismatch repair testing makes an inherited mismatch repair gene change more likely to explain the cancer.
The link to mismatch repair
A tumor called "deficient mismatch repair," or dMMR, reflects the same underlying problem. It is found with a different test. NCI genetics summaries describe two tumor tests. One is a lab stain called immunohistochemistry, which looks at repair proteins. The other is MSI testing, which compares repeat patterns in tumor DNA. Reports may use either term. Ask your team how they read your result.
Why this matters for treatment
On May 23, 2017, the FDA granted accelerated approval to pembrolizumab for MSI-H or dMMR solid tumors. The FDA calls it its first tissue/site-agnostic approval. That means it was based on the biomarker rather than where the cancer started. The approval covers certain tumors that have progressed after treatment with no satisfactory alternative, and MSI-H or dMMR colorectal cancer after certain prior drugs.
The FDA reports data from 149 patients in five single-arm trials. The objective response rate was 39.6%. For 78% of those who responded, the response lasted six months or more. Ask your team whether this or another option applies to you.
What it does not tell you
MSI-high status does not guarantee a response to immunotherapy. It also does not replace other biomarker testing. And an MSI-high result is not, by itself, a genetic test for Lynch syndrome. It is a signal to discuss whether formal genetic testing makes sense.
What a genetic counseling referral involves
If your result leads to a referral, a genetic counselor usually:
- reviews your personal and family cancer history
- explains what a positive or negative test might mean
- arranges a test that looks at the genes involved in Lynch syndrome
If a gene change is found, relatives can be offered testing for that same change. Ask the counselor what screening is advised.
What it means for your family, not just you
An MSI-high tumor does not automatically mean you carry Lynch syndrome. Only formal genetic testing can tell. Ask your team whether your relatives should be told about the result and when.
Is this urgent?
MSI testing does not need action on its own. Still, discuss the result with your team soon. It may affect your treatment options. Call your team right away if you have a fever of 100.4 F (38 C) or higher during treatment.
What to ask your team
- Is my tumor MSI-high or dMMR, or was neither found?
- Does this open up immunotherapy as an option for me?
- Should I or my family members consider genetic counseling or testing for Lynch syndrome?
- How does this result fit with any other biomarker testing I've had?
Sources
Common questions
What is microsatellite instability?
Microsatellites are short, repeating pieces of DNA. When a cell's repair system fails, errors pile up in them. A tumor with many such errors is called microsatellite instability-high, or MSI-high.
Why does MSI matter?
MSI-high tumors often respond well to immunotherapy, so the result can open up a treatment option. It can also flag a possible inherited cancer syndrome.
How is it related to MMR?
Mismatch repair (MMR) is the system that fixes these DNA errors. When it is deficient (dMMR), MSI results. The two tests measure the same underlying problem in different ways.
Does MSI-high mean I have an inherited syndrome?
Not always, but it can be a clue to Lynch syndrome. A doctor may suggest genetic counseling to look into it further.
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Written by: Cancer ExplainedSources last checked: 2026-10-02 what this meansLast updated: 2026-08-10Next planned review: 2027-07-14
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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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