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A prion (IPA /'pri??n/[1]listen&_160;(helpยทinfo)) is thought to be an infectious agent that, according to current scientific consensus, is comprised entirely of a propagated, mis-folded protein.[2] The mis-folded form of the prion protein has been implicated in a number of diseases in a variety of mammals, including bovine spongiform encephalopathy (BSE, also known as "mad cow disease") in cattle and Creutzfeldt-Jakob disease (CJD) in humans. All hypothesized prion diseases affect the structure of the brain or other neural tissue, and all are currently untreatable and are always fatal.[3] In general usage, prion refers to the theoretical unit of infection. Scientifically speaking, PrPC refers to the endogenous prion protein, which is found in a multitude of tissues, while PrPSC refers to the misfolded form of PrPC, and is responsible for the formation of amyloid plaques that lead to neurodegeneration. Prions are hypothesized to infect and propagate by refolding abnormally into a structure which is able to convert normal molecules of the protein into the abnormally structured form. All known prions induce the formation of an amyloid fold, in which the protein polymerises into an aggregate consisting of tightly packed beta sheets. This altered structure is extremely stable and accumulates in infected tissue, causing cell death and tissue damage.[4] This stability means that prions are resistant to denaturation by chemical and physical agents, making disposal and containment of these particles difficult. Proteins showing prion-type behavior are also found in some fungi and this has been quite important in helping to understand mammalian prions. However, fungal prions do not appear to cause disease in their hosts and may even confer an evolutionary advantage through a form of protein-based inheritance.[5] The word prion is a portmanteau developed by combining the first two syllables of the words proteinaceous and infectious (-on by analogy to virion)[6].
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