  We propose a simple model of pathologic microsatellite expansion, and
describe an inherent self-repairing mechanism working against expansion. We
prove that if the probabilities of elementary expansions and contractions are
equal, microsatellite expansions are always self-repairing. If these
probabilities are different, self-reparation does not work. Mosaicism,
anticipation and reverse mutation cases are discussed in the framework of the
model. We explain these phenomena and provide some theoretical evidence for
their properties, for example the rarity of reverse mutations.
