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Human prion protein. Computer model showing the molecular structure of a human prion protein. Prions are abnormal proteins that cause a group of fatal neurodegenerative diseases including BSE in cows and CJD in humans. Prions do not have a nucleic acid (RNA or DNA) genome for replication. Abnormal infectious prions directly change normal prions found in the brain to the abnormal form. The altered prion structure is extremely stable and accumulates as amyloid plaques that lead to neurodegeneration. (KEYSTONE/SCIENCE PHOTO LIBRARY/LAGUNA DESIGN)