MSI (microsatellite instability) and MSS (microsatellite stability) biomarkers indicate how stable the DNA is in a tumor.

If a cell’s DNA is unstable, particles called microsatellites are produced. Colorectal cancer tumors are often referred to as having an “MSI status,” meaning they are described as either MSI or MSS. They cannot be both.

An MSS (microsatellite stability) result indicates that the DNA in the cells are considered stable. If there is a lot of instability in the DNA, it is described as MSI-H (microsatellite instability high).

• Approximately 80-85 percent of colorectal cancer patients are classified as MSS, with a functional MMR process and no mutations in their MMR genes.
• Approximately 15 percent of colorectal patients have dMMR and areMSI-H.
• In three to five percent of colorectal patients, dMMR and MSI-H are caused by Lynch syndrome. In these patients, a hereditary mutation (germline mutation) in one of the four main MMR genes is passed from one generation to another in a family. Individuals with Lynch syndrome are at higher risk of developing colorectal, endometrial (uterine), gastric, ovarian, and other cancers.
• Lynch syndrome

MSS testing is recommended for anyone who is diagnosed with colorectal cancer.

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Knowing the details of tumor biomarkers can help you and your doctor make decisions about personalized treatment with therapies tailored specifically to the characteristics of your tumor.

Patients diagnosed with an MSS tumor have a higher risk of recurrence. If you have stage IV (metastatic) colorectal cancer, your doctor will test for additional biomarkers including KRAS/NRAS, BRAF, andHER2. 

Patients with MSS tumors are treated with fluorouracil-based chemotherapy or similar drugs.

Currently, pembrolizumab has been approved for some MSS tumors. There are ongoing clinical trials for immunotherapy in combination with different drugs for for MSS patients.

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Some of the most common side effects of treatment for patients with MSS tumors are nausea, fatigue, diarrhea or constipation, headaches, and high temperature.

It is unlikely that you will have all of these side effects, but you might have some of them. Contact your doctor immediately if you are experiencing severe symptoms.