Often caused by mistakes that are left uncorrected when DNA has been copied in a cell, microsatellite instability (MSI) is a condition that occurs in a certain type of cell, such as a cancer cell, where regions of the repeated DNA alter in length, showing instability due to a defect in mismatch repair DNA proteins. Since the errors go uncorrected, they accumulate and through DNA replication, the sequences are not preserved, ultimately creating new microsatellites.