Bovine spongiform encephalopathy (BSE) is a transmissible neurodegenerative disease. Six brain regions from 11 cattle were examined for the presence of the abnormal isoform of the prion protein (PrPBSE). The highest concentrations of PrPBSE were found in the brain stem, where the greatest degree of spongiform change was observed. Molecular cloning of the bovine PrP gene showed that it encodes a protein of 256 or 264 amino acids with five or six Gly:Pro-rich octarepeats, respectively, in contrast to all other mammalian PrP genes, which encode only five octarepeats. The bovine PrP gene is single copy, and the entire open-reading frame lies within a single exon. Since the transmission of prions across species seems to be restricted by differences in PrP sequence, the high degree of homology between sheep and bovine PrP (98%) correlates with the proposed cause of BSE.