HTT and juvenile Huntington disease: Huntington’s disease (HD), affecting ∼5–10 per 100,000 people in the Western world (Ross & Tabrizi, 2011; Mochly-Rosen et al, 2014), is a progressive neurodegenerative disease caused by a genetic mutation in the huntingtin gene, leading to the production of an expanded polyglutamine stretch in the encoded protein.