HTT and juvenile Huntington disease: Huntington’s disease (HD) is an autosomal dominant neurodegenerative disorder caused by mutations in the huntingtin (HTT) gene and characterized by the initial loss of neurons along the striatum that slowly progresses into other brain regions including the cerebral cortex and hypothalamus (Vonsattel et al., 1985; DiFiglia et al., 1997; Tebbenkamp et al., 2011).