HTT and Huntington disease: HD is considered an inherited neurodegenerative disease caused by the abnormal expansion of cytosine, adenine, and guanine trinucleotide repeats (CAG) in the Huntington gene (HTT), which is located on chromosome 4p16, leading to an abnormally long expression of polyglutamine in the HTT protein and neurodegeneration, which leads to motor, cognitive, and psychiatric symptoms.