HTT and juvenile Huntington disease: Huntington’s disease (HD) is an inherited, rare, neurodegenerative disease characterised by movement, cognitive and psychiatric symptoms.1 HD is caused by a repeat expansion of the trinucleotide cytosine-adenine-guanine (CAG) in exon 1 of the Huntingtin gene (HTT) that leads to expression of a mutant form of the Huntingtin protein.1 The greater the number of CAG repeats, the earlier the HD onset.2 A diagnosis of HD is based on the presence of significant motor abnormalities.