The prion protein (PrP) is a scrapie-associated fibril protein that accumulates in the brains of hamsters and mice infected with the scrapie agent, and also in the brains of persons affected with kuru or Creutzfeldt-Jakob disease. It has been previously proposed that PrP could be either the primary transmissible agent of scrapie or a secondary component involved in the pathogenesis of scrapie. At present, the second possibility seems more likely, for the PrP-specific mRNA is present in both infected and uninfected brains. We have isolated and sequenced the complete PrP-specific cDNA from mRNA isolated from infected mouse brains. Comparison of the mouse PrP with the hamster PrP reveals a high homology in the amino acid sequence and the presence of a conserved octapeptide repeated four times, whose function is unknown at present. Structural features are discussed and compared with other proteins. Except for its homology with the hamster PrP, mouse PrP has no significant homology to any known protein sequence, including neurofilaments, neuropeptides, and amyloid proteins of Alzheimer disease. Some features of the PrP, however, are similar to structures found in aggregating proteins, such as the wheat glutenin, keratin, and collagen.