HTT and juvenile Huntington disease: Many neurodegenerative diseases share the hallmark appearance of fibrillar protein aggregates in the brain, and while the role of these aggregates in disease remains unclear, such fibrillary structures are composed of aggregation-prone proteins, for example, mutant huntingtin (HTT) in Huntington disease, α-synuclein in Parkinson disease and amyloid-beta (Aβ) in Alzheimer disease (Scherzinger et al. 1999; Chiti and Dobson 2006; Goedert and Spillantini 2006).