Microsatelite Instability (MSI)

Microsatellites are areas of the genome comprised of mononucleotide (e.g. AAAAA) or dinucleotide (e.g. CGCGCGCG) repeats. Errors in DNA replication are more likely to occur in microsatellites during cell division. Because of the repeated sequences, the new and template DNA strands may become misaligned during DNA replication, leading to replication machinery slippage that results in too many or too few repeats in the microsatellite. The term microsatellite instability is used to describe the resulting incorrect number of microsatellite repeats.

Individuals who have Lynch syndrome are more likely to have tumors with microsatellite instability since the loss of mismatch repair gene function prevents the correction of errors that occur during DNA replication. Approximately 90% of colon tumors from individuals with Lynch syndrome demonstrate microsatellite instability, whereas only approximately 15% of sporadic colon tumors do.