The genome contains sequences consisting of 2 to 5 base repeats, shorter than minisatellite core sequences, which are thus called microsatellites. In contrast to minisatellites, microsatellites distribute evenly throughout the genome, and are thus frequently used as landmarks for gene mapping. Human hereditary diseases caused by unusual expansion of three-base repeats are now known (e.g., CGG repeats in fragile-X syndrome, CAG repeats in Huntington disease). The number of repeats varies among normal individuals, as well as among family members of the affected individuals and even among somatic cells of an individual. The inheritance pattern of the repeats does not seem to follow Mendelian rules. Patients of hereditary non-polyposis colon cancer are known to show instability in microsatellite repeat number. Radiation effects on microsatellite instability are not known.