Triplet Repeat Disease

Triplet repeat disorders fall into two subclasses, depending on the location of the trinucleotide repeat within the gene. The first subclass has triplet repeats occurring in noncoding sequences of DNA, either introns or regions at the start or end of the gene, called 5′ ("five prime") or 3′ ("three prime") untranslated regions. The second subclass is characterized by (CAG)_n repeats that code for repeated stretches of the amino acid glutamine (polyglutamine) in coding regions of the affected gene.