Scrapie is a CNS degenerative infection of sheep and goats, which is invariably fatal after incubation periods of several months to years. Related disorders are found naturally in man and other species. There is a impairment of protein catabolism in scrapie and related diseases which leads to the accumulation of sparingly-soluble protein deposits in brain. These protein aggregates may share with the amyloid of Alzheimer's disease (AD) some common stage in the biochemical pathways of their formation, although different proteins are affected in scrapie (the PrP protein) and AD (the A4-precursor protein). Recently, cattle with the clinical signs and brain pathology of a neurodegenerative disease have been reported, and this cattle disorder has been called bovine spongiform encephalopathy (BSE). BSE-affected brains contain abnormal forms of the bovine homologue of PrP. This provides biochemical evidence that BSE is cattle scrapie rather than cattle AD.