HD is associated with an expansion of CAG trinucleotide repeats in exon 1 of the huntingtin gene, which encodes for an elongated polyglutamine (PolyQ) stretch in Huntingtin protein (Htt)1, resulting in expression of mutant Htt with varying degree of CAG repeats (more than 36 to 120 repeats). Here, HTT is linked to Huntington disease.