Mismatch Repair (MMR) and Microsatellite Instability (MSI) in Gastrointestinal Cancers

Mismatch repair (MMR) and microsatellite instability (MSI) testing tell doctors whether the DNA repair system inside a cancer is working properly. When that system breaks down, cancer cells accumulate many genetic errors, and those errors are exactly what can make the cancer respond to a powerful type of treatment called immunotherapy. The same test matters for a second reason: it can reveal whether the cancer developed because of an inherited condition called Lynch syndrome, which raises cancer risk for other members of the family. This article focuses on what MMR and MSI testing mean across the gastrointestinal (digestive) cancers, including cancers of the stomach, esophagus, small intestine, pancreas, and bile ducts.

This article will help you understand what an MMR or MSI result means on a pathology report for a gastrointestinal cancer, why the test is done, how it is performed, and how the result may guide treatment decisions. A normal MMR result is not a problem, and an abnormal result has two distinct meanings that are addressed together: it may open the door to immunotherapy, and it may point to an inherited cause worth investigating for the sake of the whole family.

What the test looks for

Every time a cell divides, it copies all of its DNA, and that copying is not perfect. Small errors happen regularly. The body has a built-in repair system, mismatch repair (MMR), that finds and fixes these errors before they become permanent. It works like a spell-checker for the genetic code.

The MMR system is made up of four proteins, MLH1, PMS2, MSH2, and MSH6, that work together in pairs: MLH1 pairs with PMS2, and MSH2 pairs with MSH6. If one protein in a pair is lost or stops working, the repair function for that pair fails. When the repair system stops working inside a cancer cell, errors accumulate across the DNA, especially in short, repetitive stretches called microsatellites, which vary in length. When a laboratory test detects this pattern, the tumor is called microsatellite instability-high (MSI-H). A tumor whose repair system has broken down is called MMR-deficient (dMMR). A tumor in which the system is working normally is called MMR-proficient (pMMR) or microsatellite-stable (MSS).

These errors matter for treatment because a tumor with many errors produces many abnormal proteins that appear on the surface of the cancer cells. These look foreign to the immune system, so the body’s immune cells try to attack them. Cancers can block that attack using molecular “off switches.” Immunotherapy drugs called immune checkpoint inhibitors release those off switches, freeing the immune system to attack the cancer. Tumors that are dMMR/MSI-H tend to respond well to these drugs because the large number of errors gives the immune system many targets.

MMR and MSI by gastrointestinal cancer type

Mismatch repair deficiency can occur in any gastrointestinal cancer, but it is more common in some than in others. The sections below explain how often it appears in each, what it means for treatment, and when it points to Lynch syndrome. Colorectal cancer, where MMR/MSI testing was first developed, is covered in its own dedicated article and is summarized only briefly here.

Stomach (gastric) cancer

About 5-10% of stomach cancers are dMMR/MSI-H. These cancers tend to respond well to the immunotherapy drug pembrolizumab, which is approved in this setting. A dMMR result in a stomach cancer should prompt a discussion about Lynch syndrome, because some of these cancers are caused by an inherited MMR gene change. MMR status in stomach cancer is also considered alongside other biomarkers, such as HER2 and PD-L1, when planning treatment for advanced disease.

Esophageal and gastroesophageal junction cancer

Mismatch repair deficiency is uncommon in cancers of the esophagus and the gastroesophageal junction (the area where the esophagus meets the stomach), but it does occur more often in adenocarcinoma than in squamous cell carcinoma. When dMMR is present, it supports consideration of immunotherapy and, as in other gastrointestinal cancers, prompts an evaluation for Lynch syndrome. For junction cancers, MMR is interpreted together with HER2 and PD-L1, which often drive the main treatment decisions.

Small intestine (small bowel) cancer

Cancers of the small intestine have a relatively high rate of mismatch repair deficiency, around 14 percent, and are notably associated with Lynch syndrome. Because the small bowel is one of the cancers most strongly linked to Lynch syndrome, a dMMR result in a small intestine cancer should always lead to a discussion about inherited risk and genetic evaluation. A dMMR result also identifies patients who may benefit from immunotherapy under the approval that covers any advanced dMMR/MSI-H solid tumor.

Pancreatic cancer

Only about 1-2% of pancreatic cancers are dMMR/MSI-H. This is uncommon, but it is very important when it does occur. Pancreatic cancer is one of the most difficult cancers to treat, and standard options are limited, so identifying the small group of patients whose cancer may respond to immunotherapy matters a great deal. For these patients, pembrolizumab can sometimes produce meaningful and lasting responses. Because dMMR in pancreatic cancer is also strongly linked to Lynch syndrome, genetic testing should always be part of the conversation when this result is found.

Bile duct cancer (cholangiocarcinoma) and gallbladder cancer

Cancers of the bile ducts, called cholangiocarcinoma, and cancers of the gallbladder have a mismatch repair deficiency rate of roughly 5 to 10 percent, and Lynch syndrome accounts for a meaningful portion of these. A dMMR result in a bile duct or gallbladder cancer supports consideration of immunotherapy and warrants evaluation for Lynch syndrome.

Colorectal cancer (brief summary)

Colorectal cancer is where MMR/MSI testing began and is best understood. About 15 percent of colorectal cancers that have not spread to distant organs are dMMR/MSI-H, and roughly one quarter to one third of these are caused by Lynch syndrome. For advanced dMMR/MSI-H colorectal cancer, pembrolizumab is approved as a first-line treatment and works better than standard chemotherapy in this group. The colorectal-specific details are covered fully in a separate article.

Why is the test done?

MMR/MSI testing in gastrointestinal cancers serves three purposes, and more than one may apply to the same patient at the same time.

To find out whether immunotherapy is an option

A dMMR/MSI-H result is one of the strongest predictors that a cancer will respond to immune checkpoint inhibitor drugs. Pembrolizumab is approved for any advanced dMMR/MSI-H solid tumor that has progressed after prior treatment, an approval that covers any cancer type regardless of where it started. This is called a tumor-agnostic approval, meaning the drug is approved based on a molecular finding rather than the organ of origin. In colorectal cancer, immunotherapy is also approved in earlier treatment settings. If a gastrointestinal cancer is dMMR/MSI-H and advanced, the oncology team will discuss whether immunotherapy is appropriate.

To screen for Lynch syndrome

Finding dMMR in a tumor is the standard first step in checking for Lynch syndrome, an inherited condition that raises the risk of several cancers. Many gastrointestinal cancers, including colorectal, stomach, small intestine, pancreatic, and bile duct cancers, are part of the Lynch syndrome spectrum. Identifying Lynch syndrome matters for the whole family, because each parent, sibling, and child of a person who carries the gene change has a 50 percent chance of carrying it too. A dMMR result does not confirm Lynch syndrome on its own, but it means the possibility must be investigated.

To help understand the cancer’s behavior

In some gastrointestinal cancers, MMR status carries meaning beyond its role in guiding treatment. In early-stage colorectal cancer, a dMMR result is generally associated with a more favorable outlook and influences decisions about chemotherapy after surgery. In other gastrointestinal cancers, the main value of the result is its role in guiding immunotherapy and identifying inherited risk.

How the test is performed

MMR/MSI testing is done on tumor tissue, usually from a biopsy or tissue removed during surgery. No additional procedure is usually needed, because the tissue already collected for diagnosis is enough.

The most common first step is immunohistochemistry (IHC). A pathologist uses special stains to check whether the four MMR proteins are present in the cancer cells. Normal cells in the same sample always stain positively, which confirms the test worked. If one or more proteins are absent from the cancer cells, this indicates a mismatch repair deficiency, and the report names which proteins are missing.

DNA-based testing is the other approach. Using PCR or next-generation sequencing (NGS), the laboratory examines the tumor’s DNA directly for signs of instability. When a comprehensive molecular panel is ordered, as is increasingly routine in advanced gastrointestinal cancer, MSI status is usually calculated automatically alongside other biomarker results. Both the protein-based and DNA-based approaches test for the same underlying problem and usually agree.

How results are reported

Depending on which test was used, the pathology report will use one of these sets of terms:

MMR-proficient (pMMR) — All four proteins are present, and the repair system is intact. This corresponds to MSS (microsatellite stable) on DNA testing.
MMR-deficient (dMMR) — One or more proteins are absent, and the repair system has broken down. The report will specify which proteins are missing, for example, “loss of MLH1 and PMS2.” This corresponds to MSI-H on DNA testing.
MSS (microsatellite stable) — The DNA shows no instability. This corresponds to pMMR.
MSI-H (microsatellite instability-high) — The DNA shows significant instability. This corresponds to dMMR.
MSI-L (microsatellite instability-low) — A borderline result. For most treatment decisions, this is managed the same as MSS, though the oncologist will interpret it in the context of the other findings.

Some reports include both the protein-based and DNA-based results, particularly when one test was done to confirm or clarify the other.

What the result means

pMMR / MSS — repair system intact

A normal result means the cancer’s repair system is working. Immunotherapy based on MMR/MSI status alone is generally not recommended because checkpoint inhibitors tend not to work in pMMR/MSS tumors. Lynch syndrome is also unlikely to be the cause of the cancer. Treatment will be planned based on the cancer type, stage, and other findings.

dMMR / MSI-H — repair system broken down

An abnormal result has two implications that are addressed together. First, immunotherapy may be an option: if the cancer is advanced, pembrolizumab or another approved checkpoint inhibitor may be suitable, and the oncologist will discuss this with you. Second, Lynch syndrome should be considered: further testing is needed to determine whether the dMMR result is due to Lynch syndrome or to a non-inherited change in the tumor. This involves additional tumor tests and may lead to a referral to a genetic counselor. It does not mean Lynch syndrome has been confirmed, only that it needs to be investigated.

Telling Lynch syndrome from a sporadic result

This is the most important follow-up question after a dMMR result in any gastrointestinal cancer. Lynch syndrome is an inherited condition caused by a change in one of the four MMR genes, which are present from birth in every cell of the body. It raises the lifetime risk of several cancers, including colorectal, stomach, small intestine, pancreatic, and bile duct cancers. Each child, sibling, and parent of a person with Lynch syndrome has a 50 percent chance of carrying the same change, which is why identifying it matters for the whole family.

Whether a dMMR result is inherited or not depends on which proteins are missing and what further testing shows:

Loss of MLH1 and PMS2 together — This is the most common pattern. In most cases, it is caused by a non-inherited change called MLH1 promoter methylation, a chemical switch that turns off the MLH1 gene only within the tumor. It is not hereditary. In colorectal cancer, finding a BRAF V600E mutation alongside this pattern is further evidence of a non-inherited cause. If neither MLH1 methylation nor BRAF V600E is found, Lynch syndrome becomes more likely, and a blood test for inherited changes is recommended.
Loss of MSH2 and MSH6 together — This pattern is strongly associated with Lynch syndrome and should always prompt a genetics referral and blood (germline) testing.
Loss of MSH6 alone or PMS2 alone — These patterns are also associated with Lynch syndrome and warrant genetic evaluation.

If a blood test confirms an inherited MMR gene change, Lynch syndrome is diagnosed. For the patient, this means follow-up care is tailored to account for the higher risk of other Lynch-associated cancers, with closer surveillance and additional screening. For the family, it means biological relatives can be offered genetic testing through a process called cascade testing, so that those who carry the change can begin appropriate screening before any cancer develops. A genetic counselor guides this process. The Lynch syndrome evaluation can generally proceed at a pace that fits the treatment timeline and does not need to be resolved before cancer treatment begins.

What happens next

If the result is dMMR/MSI-H, two things proceed in parallel: the oncologist assesses whether immunotherapy is appropriate, and the care team begins evaluating for Lynch syndrome. Neither needs to wait for the other. If the result is pMMR/MSS, immunotherapy based on MMR/MSI status is not indicated, Lynch syndrome is unlikely to be the cause, and treatment is planned based on the cancer type, stage, and other findings.

Care for gastrointestinal cancers usually involves a multidisciplinary team that may include a medical oncologist, a surgeon, a gastroenterologist, a radiation oncologist, a pathologist, and, when inherited risk is being evaluated, a genetic counselor. If immunotherapy is started, patients are monitored for response and for immune-related side effects, which occur because these drugs increase immune activity throughout the body. Regular imaging and follow-up visits are used to track how the cancer is responding.

Questions to ask your doctor

Was my cancer tested for MMR/MSI status, and what was the result?
If my result is dMMR/MSI-H, which proteins are missing, and does the pattern suggest Lynch syndrome or a non-inherited cause?
Has MLH1 methylation testing or BRAF V600E testing been done to help tell Lynch syndrome from a non-inherited cause?
Should I be referred to a genetic counselor to be evaluated for Lynch syndrome?
Based on my result, am I a candidate for pembrolizumab or another immunotherapy?
How does my MMR result fit together with my other biomarker results, such as HER2 or PD-L1?
If Lynch syndrome is confirmed, what would it mean for my future follow-up and screening?
How should I share this information with my family members, and can they be tested?
Are there clinical trials I should know about based on my MMR/MSI result?
Will my treatment plan change because of this result?