1. R. S. Hegde and K. A. DeFea are in the Department of Physiology, University of California, San Francisco, CA 94143–0444, USA. J. A. Mastrianni, M. R. Scott, P. Tremblay, and M. Torchia are in the Department of Neurology, University of California, San Francisco, CA 94143–0518, USA. S. J. DeArmond is in the Departments of Pathology and Neurology, University of California, San Francisco, CA 94143, USA. S. B. Prusiner is in the Departments of Neurology and Biochemistry and Biophysics, University of...