|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.