Alzheimer’s disease (AD), Parkinson’s disease (PD), Creutzfeldt-Jakob’s disease (CJD), and tauopathies are neurodegenerative diseases presenting most commonly in aged individuals, and characterized by the accumulation of abnormal protein aggregates in the neuropil, neurons, and, in some conditions, also in glial cells, β-amyloid and hyper-phosphorylated 3R and 4R tau in AD, altered α-synuclein in PD, 3R or 4R hyper-phosphorylated tau in tauopathies, and pathogenic, mostly proteinase resistant, prion protein or prion in CJD. The gene discussed is SNCA; the disease is Alzheimer disease.