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MSI-high / dMMR colorectal cancer
If your tumor is MSI-high or dMMR, that's important, often hopeful news: immunotherapy works especially well for this group.
ℹ️ General information to help you and your loved ones talk with your care team, not medical advice. Read our disclaimer.
The quick guide
What it means: The tumor's DNA repair 'spell-checker' is broken, so it builds up many mutations that the immune system can recognize.
How it's found: A stain for four repair proteins (MMR) or a DNA test (MSI) on tumor tissue. Recommended for every colorectal cancer. Genetic testing for Lynch syndrome may follow.
How common it is: About 15% of colorectal cancers overall, and only about 4 to 5% of metastatic ones.
How treatment works
🌱 Early-stage
- Stage II dMMR colon cancer often doesn't benefit from 5-FU chemo alone.
- Immunotherapy before surgery is being used for some, especially dMMR rectal cancer, where it can avoid surgery for many.
🔁 Advanced or metastatic
- Immunotherapy such as Keytruda, or nivolumab with or without ipilimumab, as first treatment.
Approvals depend on stage and prior treatment. Your care team will decide what fits you.
Drug guides
Biomarker tests that matter
Side effects you may notice
❓ Questions you and your loved ones can ask
- Could I have Lynch syndrome?
- Should my family be tested?
- Is immunotherapy before surgery an option for me?
Back to Colorectal cancer overview. Other types: Colon cancer: non-metastatic and metastatic, Rectal cancer: early, locally advanced and metastatic, HER2-positive (HER2-amplified) colorectal cancer, KRAS G12C colorectal cancer (RAS-mutant), RAS wild-type colorectal cancer (KRAS/NRAS normal), BRAF V600E colorectal cancer.
Written by the Cancer Side Effects Guide team from FDA labels and leading cancer centers. Not medical advice. · Last updated: October 15, 2026 · How we write