Microsatellite Instability

Video Transcript

DNA is a double-stranded molecule that encodes instructions for all living things’ growth, regulation, and adaptation. Sometimes errors occur as DNA is duplicated.

The DNA code in human cells is made up of four small units called bases; adenine, cytosine, guanine, and thymine. They are represented with an A, C, G, and T. The two strands of DNA are not identical; rather, they complement one another. The complementary bases on either side of the DNA strand are called “base pairs.” Adenine is paired with thymine and cytosine is paired with guanine.

As a cell prepares to divide itself, it must first duplicate its DNA. To do this, the DNA is unwound and split apart so that new base pairs can be matched to the two original strands. As a result, two identical sets of double-stranded DNA molecules are created.


Specific areas of DNA are short, repeated sections of the genetic code called microsatellites. Microsatellites usually comprise 6-10 base pairs and can occur thousands of times throughout the entire DNA code. Sometimes, the bases are matched incorrectly during duplication. For example, adenine is paired with cytosine or guanine instead of thymine. This can happen if the two DNA strands become misaligned.


The human body has a mismatch repair system that identifies when bases are mismatched or paired incorrectly and corrects these mistakes. However, sometimes, cancer cells can have defects in this repair system, and DNA alignment errors cannot be fixed. Microsatellite Instability or MSI occurs when the mismatch repair system is not working correctly, and the misaligned DNA changes the length of microsatellite regions. The presence of microsatellite instability in a tumor can change the type of therapy your doctor chooses to provide.


Caris performs next-generation DNA sequencing that detects mutations in all genes associated with mismatch repair, including microsatellite instability. The more information your doctor has about your specific tumor, the more effective your treatment can be.

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Patient Education Videos

The Central Principles of Molecular Biology video series is meant to help explain molecular profiling and cancer, so that you can understand how Caris molecular profiling helps fight cancer through precision oncology.