In a large number of neurodegenerative diseases, the accumulation of misfolded proteins is a common and toxic feature; such proteins include amyloid β (Aβ), which produced by the processing of amyloid precursor protein (APP) in Alzheimer’s disease (AD); polyglutamine (polyQ), a product of CAG repeat expansions in polyQ diseases such as Huntington’s disease (HD); and the mutant form of superoxide dismutase 1 (SOD1), which is found in familial amyotrophic lateral sclerosis (ALS). This evidence concerns the gene APP and juvenile Huntington disease.