In all people, the three-letter codon sequence C-A-G is repeated several times at one end of the Huntington gene. In people with HD, the Huntington gene has an increased number of CAG repeats. Thus, there are different versions or alleles of the Huntington gene, one for each different number of CAG repeats. For simplicity within this website, the term "non-HD allele" is used to refer to any allele of the Huntington gene containing a number of CAG repeats within the normal range (10-35). The term "HD allele" is used to refer to any allele of the Huntington gene with an extended number of CAG repeats.
Last Modified: 9-13-02