These amyloid fibrils are composed of aggregation-prone proteins, such as 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; See Table 1 for a list of aggregation-prone proteins involved in neurodegenerative diseases). The gene discussed is SNCA; the disease is Parkinson disease.