Bovine spongiform encephalopathy (BSE) in cows, scrapie in sheep, and Creutzfeldt-Jakob disease (CJD) in humans are examples of prion diseases. The central event in the pathogenesis of these fatal disorders is hypothesized to be the post-translational conversion of a normal host protein of unknown function, termed PrPC into an abnormal isoform called PrPSc. The idea that protein alone can carry information sufficient to ensure its own propagation was an unprecedented challenge to the "central dogma" of molecular biology which essentially states that nucleic acids, not proteins, are the biological information carriers. The work that led to current understanding of prion diseases originated more than four decades ago. In the 1960s, Tikvah Alper and her co-workers reported that the scrapie agent was extremely resistant to treatments that normally destroy nucleic acids, but sensitive to procedures...