A decade after the discovery of viroids, Stanley Prusiner made the startling claim that scrapie, a neurodegener-ative disease of sheep, was caused by a self-replicating agent composed solely of protein. He called this type of entity a prion, and in the years which followed, other, related, diseases of humans and animals were shown to have a similar cause. These include bovine spongiform encephalopathy (BSE, 'mad cow disease') and its human equivalent, Creutzfeldt-Jakob disease.
How could something that contains no nucleic acid be capable of replicating itself? -Prusiner's idea seemed to go against the basic rules of biology. It appears that prions may be altered versions of normal animal proteins, and somehow have the ability to cause the normal version to refold itself into the mutant form. Thus the prion propagates itself. All prion diseases described thus far are similar conditions, involving a degeneration of brain tissue.
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