1. Neurobiology division, MRC Centre, Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, England U.K, Laboratory of Molecular Biophysics, University of Oxford, Rex Richards Building, South Parks Road, Oxford OX1 3QT, England U.K, and Centre for Research into Neurodegenerative Diseases, and Department of Medical Biophysics, Tanz Neuroscience Building, 6 Queens Park Crescent W, Toronto, Ontario M5S 3H2, Canada