HTT and juvenile Huntington disease: Huntington’s disease (HD) is a fatal autosomal dominant neurodegenerative disorder characterized by an expanded trinucleotide CAG (cytosine-adenine-guanine) repeat in exon 1 of the HTT (huntingtin) or IT15 gene (located at 4p.16.3) (MacDonald, 1993; Kerkis et al., 2022; Jiang et al., 2023).