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.
Trinucleotide repeat expansion occurring in a parental germline cell can lead to children that are more affected or display an earlier onset and greater severity of the condition.
In general, several neurodegenerative disorders were found to involve nucleotide repeat expansions in protein coding sequences.