Trinucleotide repeats are tandem repetitions of polymorphic (many forms) triplets of nucleotides. The spreading out of these repeats from a low to a high copy number is referred to as trinucleotide expansion, a process that leads to genetic instability and is most frequently associated with the emergence of serious diseases.

Simple nucleotide repeats (dinucleotide, trinucleotide, and tetranucleotide repeats) make up a substantial fraction of the eukaryotic genome. The high level of polymorphisms in these repeats has been very useful in genome analysis. The structures known as microsatellites and minisatellites established by these repeats helped facilitate forensic studies, mapping and positional cloning of disease related genes, and analysis of phylogenetic data. The full significance of these repeats, besides their involvement in certain diseases, has not been understood. Trinucleotide repeats are most frequently involved in disease association in human. The distribution of trinucleotide repeats in the eukaryotic genome...