The trinucleotide repeat disorders (also known as trinucleotide repeat expansion disorders or triplet repeat expansion disorders) are a set of genetic disorders caused by an increase in the number of trinucleotide repeats in certain genes exceeding the normal, stable, threshold. Each gene affected by trinucleotide repeat expansion has a different number of repeats that constitutes the normal threshold and the number that results in manifestation of disease.
The trinucleotide repeat disorders are divided into two categories determined by the type of repeat. The most common repeat is the triplet CAG which when present in the coding region of a gene codes for the amino acid glutamine (Q). Therefore, these disorders are referred to as the polyglutamine (polyQ) disorders. The remaining disorders either do not involve the CAG triplet or the CAG triplet is not in the coding region of the gene and are, therefore, referred to as the non-polyglutamine disorders.