Creutzfeldt-Jakob disease (CJD) of humans and scrapie of animals are degenerative, transmissible neurologic diseases caused by prions. The only known macromolecules within prions are prion proteins (PrP). The cDNA encoding the hamster prion protein (PrP 27-30) has been cloned and sequenced (Oesch et al., 1985). Using that hamster PrP cDNA, we screened a human retina cDNA library and sequenced the cDNA clone with the longest hybridizing insert. This insert was found to contain a long open reading frame (ORF) encoding the human prion protein. Northern transfer analysis showed that a related poly(A)+RNA measuring approximately 2.5 kb is expressed in a variety of human neuroectodermal cell lines. Human PrP differed from hamster PrP at 27 of 253 amino acids and at 98 of 759 ORF nucleotides. Conservation of PrP amino acid sequence between hamster and human is nearly 90%, reflecting similar structural features and shared antigenicity of the two proteins (Bockman et al., 1985). The human PrP sequence contained a presumptive amino-terminal signal peptide of 22 amino acids, two hydrophobic segments of sufficient length to span membranes, and two possible sites for N-glycosylation. The conservation between the hamster and human prion proteins suggests that they may have an important role in cellular metabolism and may explain the similarities between scrapie and CJD.