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Prion protein. Computer model of the prion protein responsible for causing the diseases BSE (bovine spongiform encephalopathy) in cows and CJD (Creutzfeldt-Jakob disease) in humans. It is a mutated form of a normal cell protein (PrP). This prion comprises spiralling alpha helices (orange) and straight beta sheets (green). The C and N represent the carbon and nitrogen ends of the amino acid structure respectively. Both BSE and CJD are brain-wasting disorders characterised by progressive dementia and a rapid deterioration in co-ordination, leading inevitably to death. (KEYSTONE/SCIENCE PHOTO LIBRARY/PASIEKA)