An abnormal form of a normal cell protein (PrP) found in the brain of mammals that is the agent responsible for the diseases scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle, and Creutzfeldt-Jakob disease in humans. Produced by mutation of the normal PrP gene, the abnormal prion protein induces the normal protein to fold incorrectly, causing it to form aggregates. These accumulate in the brain and progressively damage and destroy brain cells. Prion protein can be transmitted to other individuals of the same or closely related species, by injection or ingestion of infected tissue, and is transmissible between species that are not closely related, e.g. between cattle and humans.