Huntington disease (HD)

HD is a progressive fatal neurodegenerative disease. Like
FRAX–A, HD is caused by a triplet repeat expansion. The HD
expansion involves a CAG triplet in exon 1 of the IT15 gene on
chromosome 4. The expansion is translated into a
polyglutamine tract in the huntingtin protein gene product
that is believed to cause a dominant gain of function leading to
neuronal loss.
In normal individuals, the CAG unit in exon 1 has between
9 and 35 repeats. Affected individuals have repeats of 36 units
or greater, with over 90% of affected subjects having 40–55
repeats. In general, the greater the number of repeats an
individual has, the earlier the age of onset will be, although this
relationship is stronger for higher repeat numbers.