Prion diseases such as scrapie in sheep, bovine spongiform encephalopathy in cattle, Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker disease and fatal familial insomnia in man are neurodegenerative disorders. In humans, the diseases can be sporadic, inherited, or acquired by infection. The underlying pathogenic event in prion diseases is a conformational modification of the cellular isoform prion protein (PrP(C)) to the pathogenic isoform (PrP(SC)) that accumulates in the central nervous system. However, in humans, in some inherited cases the pathological PrP is not PrP(Sc), but a transmembrane form of prion protein, (Ctm)PrP. Prion protein is encoded by the cellular gene, PRNP, which has been mapped to human chromosome 20p21. Familial prion diseases are thought to result from a change in structure of the prion protein produced by the mutated PRNP gene. Furthermore, polymorphic codon 129 of the PRNP gene encodes either methionine or valine and appears to influence the susceptibility of patients to iatrogenic and sporadic CJD as well as the neuropathological phenotype in these forms of CJD. Polymorphisms in the promoter region of PRNP gene or disturbances in prion protein metabolism, such as incorrect activity of cellular chaperones or proteasomes are considered as susceptibility factors in human prion diseases.