HTT and juvenile Huntington disease: Huntington’s disease (HD) is a progressive neurodegenerative disorder caused by the anomalous expansion of polyglutamine repeats (encoded by the trinucleotide CAG, Cytosine-Adenine-Guanine) in the first exon of the huntingtin gene (HTT, also known as HD or IT15 gene), located in the short arm of chromosome 4 (4p16.3) (MIM#143100) [1].