The prion protein (PrP) plays an essential role in the pathogenesis of a group of sporadic, genetically determined and infectious fatal degenerative diseases, referred to as "prion diseases", affecting the central nervous system of humans and other mammals. The cellular PrP is encoded by a single copy gene, highly conserved across mammalian species. In prion diseases, PrP undergoes conformational changes involving a shift from alpha-helix to beta-sheet structure. This conversion is important for PrP amyloidogenesis, which occurs to the highest degree in the genetically determined Gerstmann-Sträussler-Scheinker disease (GSS) and prion protein cerebral amyloid angiopathy (PrP-CAA), while it is less frequently seen in other prion diseases. GSS and PrP-CAA are associated with point mutations of the prion protein gene (PRNP); these conditions show a broad spectrum of clinical presentation, the main signs being ataxia, spastic paraparesis, extrapyramidal signs and dementia. In GSS, parenchymal amyloid may be associated with spongiform changes or neurofibrillary lesions; in PrP-CAA, vascular amyloid is associated with neurofibrillary lesions. A major component of the amyloid fibrils in the two diseases is a 7 kDa peptide, spanning residues 81-150 of PrP.