A multitude of genes, in various eukaryotic species (including humans), contain a number of repetitions of the nucleotide triplet CAG or CAA. When the gene is translated into a protein, each of these triplets gives rise to a glutamine unit, resulting in a polyglutamine tract. Different alleles of such a gene often have different numbers of triplets since the highly repetitive sequence is prone to contraction and expansion.
Several inheritable neurodegenerative disorders, the polyglutamine diseases, occur if a mutation causes a polyglutamine tract in a specific gene to become too long. Important examples of polyglutamine diseases are spinocerebellar ataxia and Huntington's disease. It is believed that cells cannot properly dispose of proteins with overly long polyglutamine tracts, which over time leads to damage in nerve cells. The longer the polyglutamine tract, the earlier in life these diseases tend to appear.
- Laura Bonetta, Polyglutamine Diseases: A Devastating Genetic Stutter, Howard Hughes Medical Institute. Retrieved 30 December 2008.