MSI

Meaning of MSI: MSI marks a defect in a cell's DNA repair machinery, seen as changes in short repeated DNA sequences, most notably in colorectal and endometrial tumors.

Definition of MSI

MSI, or microsatellite instability, is a change detected in the DNA of certain cells in which the number of repeats within microsatellites, short sequences of DNA that repeat multiple times in a row, differs from the number of repeats present in the DNA a person inherited. Microsatellites are especially prone to copying errors during cell division.

MSI typically arises when a cell loses the normal ability to repair mistakes made while copying DNA, a system called mismatch repair. When mismatch repair is impaired, errors in microsatellite regions accumulate rather than being corrected, producing the pattern recognized as MSI.

MSI is tested for most often in colorectal and endometrial tumors, where a high level of MSI can indicate an underlying mismatch repair defect, including certain inherited conditions such as Lynch syndrome, and this finding can also help guide decisions about which treatments, including certain immune based therapies, a tumor is more likely to respond to.