Huntington's Disease Curve (IMAGE) Washington University in St. Louis Caption In the figure above, the open circles are data for individual patients with Huntington's disease, and the solid circles are averages for a given CAG repeat length. Huntington's disease is caused by a defect in the huntingtin gene (Htt) that causes an abnormal expansion in the CAG codon or triplet that codes for the amino acid glutamine. This CAG triplet expansion in unrelated genes is the root of at least nine neurodegenerative disorders, including Huntington's disease. Credit MacDonald et al., (2003) Neuromolecular Med. 4: 7-20 Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.