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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The full explanation.
What the term describes
Microsatellites are short, repeating stretches of DNA scattered through our genes. Cells normally fix small copying errors in them. Microsatellite instability, or MSI, means those errors are not being repaired and have built up in the tumor.
MSI-high and what it signals
When many errors accumulate, a tumor is called MSI-high. This tells doctors two useful things. First, MSI-high tumors often respond well to immunotherapy, so the result can open a treatment path. Second, it can be a clue that an inherited condition — most often Lynch syndrome — is present.
The link to mismatch repair
MSI is caused by a breakdown in the mismatch repair (MMR) system, the cellular machinery that fixes these errors. A tumor described as "deficient mismatch repair," or dMMR, is essentially the same finding measured a different way. Reports may use either term.
What to do with the result
If your report shows MSI-high or dMMR, ask your team what it means for treatment options and whether genetic counseling makes sense. It is often good news for immunotherapy and worth exploring further.
Words to know
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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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