MSI-H Colorectal Cancer: Meaning

This page is for people whose colorectal cancer report says MSI-H or dMMR. It explains what the result means, how labs test for it, why it matters for treatment and for your family, and when to get help.

Microsatellites are short pieces of DNA where a few letters repeat over and over. Cells copy them clumsily. A repair crew normally fixes the slips.

That crew is the mismatch repair system, and four proteins do most of the work: MLH1, MSH2, MSH6, and PMS2. A fifth gene, EPCAM, can switch off MSH2.

When the crew fails, those repeats end up with the wrong number of copies. That is microsatellite instability. When many sites are affected, the tumor is called MSI-H, meaning microsatellite instability-high.

You will also see dMMR, short for mismatch repair deficient. In practice the two labels point at the same broken system. One names the cause, the other names the fingerprint it leaves.

Labs get there by two routes, and your report will name one or both.

Immunohistochemistry (IHC) stains the tumor for the four proteins. All four present is normal. One or more missing means dMMR, and which protein is missing is a clue to why.

MSI testing reads the repeat sequences themselves, by PCR (a lab method that copies DNA so it can be read) or by gene sequencing. It counts how many are unstable.

Both are accepted. In the trial that changed first-line treatment, MSI-H or dMMR status was determined locally by either PCR or IHC.

If your pathology report mentions only one method, that is normal. If it mentions neither, ask whether the test was done at all.

Finding dMMR is not the end. The next question is whether the cause is inherited.

Lynch syndrome is the inherited version. It comes from a germline change in MLH1, MSH2, MSH6, PMS2, or EPCAM. It is the most common inherited colorectal cancer syndrome. NCI says it accounts for about 3% of newly diagnosed colorectal cancers. All high-penetrance inherited genes together explain only 5% to 6% of cases.

Most MSI-H tumors are not inherited. In sporadic cases, the MLH1 gene is silenced by hypermethylation of its promoter. That is a chemical switch. It shuts the gene off in the tumor alone.

Two follow-on tests sort this out. MLH1 promoter methylation analysis points toward sporadic disease. So does a BRAF pathogenic variant. It is absent in colorectal cancers from people with Lynch syndrome. It is rare even in sporadic adenomatous polyps.

NCI's summary supports universal tumor screening. That means testing every new colorectal cancer, whatever the person's age, and whether or not they meet the older family-history criteria for Lynch syndrome.

For advanced disease, MSI-H moves you off standard chemotherapy. It moves you onto immune checkpoint inhibitors, drugs that release a brake on the immune system.

The pivotal trial was KEYNOTE-177. It enrolled 307 people with untreated MSI-H or dMMR colorectal cancer that could not be removed or had spread. Half got pembrolizumab by vein every 3 weeks. Half got chemotherapy, either mFOLFOX6 or FOLFIRI, with or without bevacizumab or cetuximab.

Median progression-free survival was 16.5 months with pembrolizumab. It was 8.2 months with chemotherapy. The hazard ratio was 0.60, with a 95% confidence interval of 0.45 to 0.80. The two-sided p value was 0.0004. The FDA approved pembrolizumab as first-line treatment for this group on June 29, 2020. At the final analysis, the difference in overall survival was not statistically significant. The label notes that 60% of people in the chemotherapy group later switched to this kind of immunotherapy. It is given by drip either every 3 weeks or, at a larger amount, every 6 weeks, whichever suits the clinic and you.

On April 8, 2025, the FDA approved nivolumab with ipilimumab for the same setting. It covers adults and children 12 and older whose MSI-H or dMMR colorectal cancer cannot be removed or has spread.

The trial was CHECKMATE-8HW, a three-arm open-label study. Nobody in it had received immunotherapy before. In the first-line comparison against chemotherapy, median progression-free survival was not reached with the combination. It was 5.8 months with chemotherapy. The hazard ratio was 0.21, with a 95% confidence interval of 0.14 to 0.32.

The schedule pairs nivolumab with ipilimumab every 3 weeks for four rounds, then continues nivolumab alone every 4 weeks. The amounts are fixed by the protocol and prepared for you.

Two active options now exist for the same biomarker, and their side effects differ. Ask your team how they compare for you. That is a real conversation, not a formality.

On May 23, 2017, pembrolizumab became the first drug the FDA approved based on a biomarker rather than the organ the cancer started in. The indication covered adults and children with MSI-H or dMMR solid tumors that had grown after prior treatment and could not be removed or had spread.

The evidence came from 149 patients across five uncontrolled single-arm trials. Ninety had colorectal cancer and 59 had one of 14 other cancer types. NCI reports that about 40% of these patients had measurable tumor shrinkage. Among people who responded, 78% had responses lasting six months or longer. This was an accelerated approval, which means later studies had to confirm the benefit.

That is why an MSI-H result on a colorectal specimen is treated as important information even outside colorectal treatment plans.

• Whether MSI or MMR testing was done, and by which method
• If IHC was used, which of the four proteins were present and which were absent
• If MLH1 was absent, whether MLH1 promoter methylation testing was done
• Whether BRAF testing was done
• Whether germline genetic testing for Lynch syndrome has been ordered or completed
• Whether a genetic counselor has been involved

If the tumor result points toward Lynch syndrome and germline testing confirms it, the finding is no longer only about this cancer.

It changes your own follow-up for other cancers. It also gives first-degree relatives a specific gene to test for, rather than a vague family history. First-degree relatives are parents, siblings, and children. Testing them is the point of finding it.

If testing points to sporadic disease instead, that is genuine information too. It closes the question for your family and keeps the focus on treatment.

Either way, this is one of the few pathology findings in colorectal cancer that changes three things at once: drug choice, follow-up, and family screening. Read the actual report, not a summary line.

The result itself is a lab finding, not an emergency. What it leads to is not. Checkpoint inhibitors work by releasing a brake on the immune system, so the immune system can inflame healthy organs. The list below follows MedlinePlus's list of serious pembrolizumab side effects, which it says need a call to your doctor immediately or emergency care.

• Call 911 or go to an emergency department if you have chest pain, a new or worsening cough, or trouble breathing while on pembrolizumab or nivolumab with ipilimumab. Lung inflammation is one of the reactions these drugs cause. Do not drive yourself.
• Call your care team right away if you have diarrhea that is more frequent than usual, blood or mucus in your stools, or severe stomach pain. Inflammation of the bowel is the reaction most often seen on these drugs, and it is treated with steroids rather than anti-diarrhea medicine.
• Call your care team right away if your skin or the whites of your eyes turn yellow, or your urine goes dark. That points to the liver.
• Call your care team right away if you have confusion, memory problems, a stiff neck, new weakness, or blistering and peeling skin.
• Call your care team right away if you cannot pass stool or gas, are vomiting repeatedly, and your abdomen is swollen. A colorectal tumor can block the bowel. MedlinePlus says to get help right away for belly pain during cancer treatment, or when you cannot pass stool and are vomiting.
• Call your care team right away if you feel unusually tired, thirsty, or cold, or you have a new rash, joint pain, or blurred vision. Checkpoint inhibitors can affect hormone glands and other organs. These signs can build slowly, but MedlinePlus still lists them among the ones to call about immediately.

• https://www.fda.gov/drugs/drug-approvals-and-databases/fda-approves-pembrolizumab-first-line-treatment-msi-hdmmr-colorectal-cancer
• https://www.fda.gov/drugs/resources-information-approved-drugs/fda-approves-nivolumab-ipilimumab-unresectable-or-metastatic-msi-h-or-dmmr-colorectal-cancer
• https://www.cancer.gov/news-events/cancer-currents-blog/2017/fda-pembrolizumab-genetic-features
• https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=9333c79b-d487-4538-a9f0-71b91a02b287
• https://medlineplus.gov/ency/article/003120.htm
• https://www.cancer.gov/types/colorectal/hp/colorectal-genetics-pdq
• https://www.cancer.gov/types/colorectal/hp/colon-treatment-pdq
• https://medlineplus.gov/druginfo/meds/a614048.html
• https://medlineplus.gov/ency/article/000260.htm