Since their discovery in 1991, triplet repeat mutations have been found to be the cause of genomic fragile sites, two of which are linked to mental retardation, myotonic dystrophy, and several late-onset neurodegenerative diseases. In all cases, these mutations exhibit gametic and/or somatic instability once they have expanded into the mutant range. The mutations are located in coding and noncoding gene regions and have been found to act by dominant and recessive mechanisms. A wide range of mouse models has been generated to understand both of the mechanisms that underlie repeat instability and the molecular pathogenesis of the diseases. Mouse models have proved extremely useful in these goals and are now also being used for the preclinical testing of therapeutic compounds. This chapter reviews the successes and limitations of the approaches that have been developed.
