Microsatellite and trinucleotide-repeat evolution: evidence for mutationalbias and different rates of evolution in different lineages

Microsatellites are stretches of repetitive DNA, where individual repeat un its comprise one to six bases. These sequences are often highly polymorphic with respect to repeat number and include trinucleotide repeats, which are abnormally expanded in a number of diseases. It has been widely assumed th at microsatellite loci are as likely to gain and lose repeats when they mut ate. In this review we present population genetic and empirical data arguin g that microsatellites, including normal alleles at trinucleotide-repeat di sease loci, are more likely to expand in length when they mutate. In additi on, our experiments suggest that the rates of expansion of such sequences d iffer in related species.