also called MSI. A type of genetic mutation often linked to hereditary non-polyposis colon cancer (HNPCC). This mutation causes size differences in sections of deoxyribonucleic acid (DNA) that are normally the same size in all cells. Testing for MSI is done on tissue taken from the cancer to find out if this DNA is of different lengths; if it is, HNPCC genetic testing is usually offered. See also deoxyribonucleic acid, genetic testing, hereditary non-polyposis colon cancer, mutation.