HTT and juvenile Huntington disease: Huntington’s disease (HD), described by George Huntington in 1872, is an infrequent (5–10 cases per 100,000 inhabitants in Western Europe, Venezuela, and the United States, with a lower prevalence in Asian countries) neurodegenerative neuropsychiatric genetic disorder caused by an autosomal dominant mutation consisting of the pathogenic expansion of the trinucleotide repeat cytosine-adenine-guanine (CAG) in the huntingtin gene (HTT or IT-15, chromosome 4p16.3, gene ID 3064, MIM 613004) [1,2,3,4].