1. D. L. Price is in the Departments of Pathology, Neurology, and Neuroscience and the Neuropathology Laboratory, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA. S. S. Sisodia is in the Department of Pharmacological and Physiological Sciences, University of Chicago, Chicago, IL 60637, USA. D. R. Borchelt is in the Department of Pathology and the Neuropathology Laboratory, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.