Prion protein. Computer model of molecules of the prion protein responsible for causing the diseases BSE (bovine spongiform encephalopathy) in cows and CJD (Creutzfeldt-Jakob disease) in humans. This is a mutated form of a normal cell protein (PrP). The prion consists of spiralling alpha helices (orange) and straight beta sheets (green). Both BSE and CJD are brain-wasting disorders characterised by progressive dementia and a rapid deterioration in co-ordination, leading inevitably to death.