The mutation associated with Huntington’s disease (HD) was isolated in 1993, mapped to the short arm of chromosome 4. This historic discovery was made by an international collaboration led by a team of neuroscientists in a laboratory in the Charlestown Navy Yard. Normally, the trinucleotide cytosine-adenine-guanine (CAG) is repeated within exon 1 of the Huntingtin gene thirty-five times or fewer. The mutated gene has thirty-six or more CAG repeats. This expanded genetic stutter results in too many glutamines in the Huntingtin protein and causes the disease.
Every child of a parent with HD has a 50 percent chance of inheriting the mutated gene. The discovery of this mutation made genetic testing possible for anyone living at risk. The test definitively determines genetic status. A positive test result means the person has the mutation and will develop HD. To date, 90 percent of people at risk for HD choose not to know.