Understanding MMR, MSI-H and PD-L1: Three Clues That Help Guide Cancer Care

If your pathology report mentions MMR, MSI or PD-L1, it is describing features of your cancer that may help your oncology team choose treatment. These are related to some immunotherapy decisions, but they measure different things. They are not general blood tests for finding cancer, and a result alone cannot tell you which treatment is right for you. NCI: Biomarker testing

1. MMR: checking the DNA repair proteins

Mismatch repair, or MMR, is one of the cell’s systems for correcting mistakes made when DNA is copied.

A laboratory can use immunohistochemistry, or IHC, to stain a tumor sample for four repair proteins: MLH1, PMS2, MSH2 and MSH6. A pathologist examines the staining, including control cells that show whether the test worked.

• pMMR, or mismatch repair proficient: the expected proteins are retained on a technically adequate test.
• dMMR, or mismatch repair deficient: the staining pattern shows loss of one or more repair proteins and supports a problem with the repair system.

The pattern of missing proteins can guide further testing. IHC checks protein expression; it does not directly sequence the genes or prove that every retained protein works normally. CAP: MMR/MSI testing guideline; NCI: Colorectal cancer genetics

2. MSI: looking for DNA-copying errors

Microsatellites are short, repeated stretches of DNA. When mismatch repair does not work properly, their lengths can change. This is called microsatellite instability, or MSI.

MSI testing examines DNA, commonly using PCR or a suitably validated next-generation sequencing, or NGS, test.

• MSI-H: high microsatellite instability.
• MSS: microsatellite stable on that test.
• Some reports also use MSI-L, meaning low instability. This is not the same as MSI-H.

dMMR and MSI-H often go together, but they are not identical test results. One evaluates proteins; the other evaluates DNA changes. Results can disagree because of tumor biology or testing limitations. A pathologist may recommend review or an additional method. The appropriate method depends on the cancer type; not every NGS panel includes validated MSI testing. CAP: MMR/MSI testing guideline; CAP: When two cancer tests do not match

3. PD-L1: checking a protein involved in immune signaling

PD-L1 is a protein that can interact with PD-1 on immune cells and reduce their activity. Some cancers use this interaction to help avoid an immune attack. Drugs called immune checkpoint inhibitors can block these signals, helping immune cells attack cancer. NCI: Immune checkpoint inhibitors

PD-L1 is usually measured by IHC on tumor tissue, using an assay and scoring system appropriate for the cancer and treatment being considered.

• TPS, or tumor proportion score: the percentage of living tumor cells with the required PD-L1 staining.
• CPS, or combined positive score: PD-L1-staining tumor cells and specified immune cells, divided by the number of living tumor cells, multiplied by 100. CPS is a score, not simply a percentage of all cells.

TPS and CPS cannot be substituted for each other. Different assays and treatment settings may use different thresholds, so “PD-L1 positive” needs context. Ask which assay, score and cutoff your report uses. FDA: PD-L1 assay and scoring definitions; FDA: Companion diagnostic tests

What sample is needed?

These tests are often performed on tissue already collected during a biopsy or cancer surgery. The laboratory checks whether enough suitable tumor is present. Sometimes another sample is needed; a new biopsy is not automatically required. NCI: Biomarker testing

Some blood-based liquid biopsies can assess MSI by analyzing tumor DNA circulating in the blood. They do not automatically replace tissue testing. A tumor may release too little DNA for a blood test to detect its features, so a negative blood result may require tissue testing. Standard tumor-DNA blood panels generally do not provide the tissue-based MMR or PD-L1 IHC results described here. The Jackson Laboratory: Liquid biopsy testing

How can the results affect treatment?

The meaning depends on the cancer, its stage, previous treatment, other biomarkers, your health and the approvals where you receive care. Examples include:

• Colorectal cancer: MSI-H or dMMR can support checkpoint immunotherapy in particular settings, including certain cancers that have spread or cannot be removed surgically. FDA: Pembrolizumab in MSI-H/dMMR colorectal cancer
• Endometrial cancer: MMR/MSI status helps characterize the cancer and guide care. Some immunotherapy options depend on these results; others do not. For example, a US-approved dostarlimab-plus-chemotherapy regimen for primary advanced or recurrent endometrial cancer is not restricted to dMMR/MSI-H tumors. FDA: Endometrial cancer indication
• Stomach and gastroesophageal junction cancer: PD-L1 CPS, MSI/MMR and other markers can inform different treatment decisions. One US-approved pembrolizumab combination for advanced HER2-positive disease requires PD-L1 CPS of at least 1. That threshold does not apply to every cancer or regimen. FDA: Gastric cancer indication
• Non-small cell lung cancer: PD-L1 TPS helps guide certain immunotherapy choices. Gene findings such as EGFR or ALK and the planned treatment combination also matter. NCI: Pembrolizumab

An MSI-H/dMMR result can also support certain treatment options across different solid-tumor types, subject to the specific drug indication. A PD-L1-negative or pMMR/MSS result does not rule out every immunotherapy regimen. NCI: Pembrolizumab

Does dMMR mean the cancer is inherited?

Not necessarily. MMR problems can develop only in the tumor. In some people, the findings raise the possibility of Lynch syndrome, an inherited condition that increases the risk of several cancers.

The care team may recommend additional tumor testing, genetic counseling and, when appropriate, a separate inherited-risk test. MLH1 promoter methylation testing, and BRAF testing in certain colorectal cancer workups, can help clarify the cause. Tumor testing alone does not diagnose Lynch syndrome. Personal and family history still matter. NCI: Colorectal cancer genetics

Important limits to remember

Tests can miss a finding or be inconclusive. Sample quality, the amount of tumor and differences between parts of a cancer can affect results. A favorable biomarker does not guarantee that a treatment will work. Immunotherapy can also cause serious immune-related side effects, so the decision includes both expected benefit and risk. NCI: Biomarker testing; NCI: Immune checkpoint inhibitors

Questions to take to your appointment

1. Which tests were done, on which sample, and what are my exact results?
2. Is the result reliable for my cancer type, or is another test needed?
3. Which treatment options does it change for my stage of cancer?
4. Do I need genetic counseling or testing for Lynch syndrome?
5. Can I have a copy of my pathology and biomarker reports?

Patient takeaway: MMR, MSI and PD-L1 provide different clues. Your oncology team puts those clues together with the rest of your medical information to recommend care.

General education, not a personal diagnosis or treatment recommendation. Drug examples describe US FDA indications; local approvals and access vary. Sources checked November 6, 2026.

#EYI #CancerEducation #BiomarkerTesting #MMR #MSI #PDL1 #Immunotherapy #PrecisionOncology #PatientEducation

Sources and further reading

• NCI: Biomarker testing
• CAP: MMR/MSI testing guideline
• NCI: Colorectal cancer genetics
• CAP: When two cancer tests do not match
• NCI: Immune checkpoint inhibitors
• FDA: PD-L1 assay and scoring definitions
• FDA: Companion diagnostic tests
• The Jackson Laboratory: Liquid biopsy testing
• FDA: Pembrolizumab in MSI-H/dMMR colorectal cancer
• FDA: Endometrial cancer indication
• FDA: Gastric cancer indication
• NCI: Pembrolizumab