HTT and Huntington disease: On the other hand, HD shows autosomal dominance with full penetration, which is caused by a CAG repeat expansion (> 36 CAGs) in exon 1 of the HD gene that is translated to a polyglutamine (polyQ) repeat in the disease protein huntingtin (HTT) [10, 11].The polyQ expansion causes HTT to misfold and aggregate in the patient brain, resulting in the preferential loss of the medium spiny neurons in the striatum and extended neurodegeneration in various brain regions as HD progresses [11].