Huntington disease (HD) is an autosomal dominant, neurodegenerative disorder, which is genetically characterized by a pathologically expanded CAG repeat sequence (>39) in exon 1 of the IT15 gene located on chromosome 4 with a strong correlation between the CAG repeat length and the age of disease onset (Snell et al., 1993; The Huntington’s Disease Collaborative Research Group, 1993). Here, HTT is linked to Huntington disease.