Caption: Prion proteins. Light micrograph of a section through prion protein fibrils from E. coli (Escherichia coli) bacteria. Prions are misfolded proteins that occur in the brains of people with degenerative brain disorders such as Alzheimer's disease and Parkinson's disease. They have the ability to transmit their misfolded shape onto normal variants of the same protein. Recombinant proteins such as the prion protein shown here are often used to model how proteins misfold and sometimes polymerise in these neurodegenerative disorders.