1. Department of Molecular Biology, Genentech, South San Francisco, CA 94080, USA.
2. Department of Neurology, University of California at San Francisco, San Francisco, CA 94158, USA.
3. Department of Cell and Molecular Biology, Karolinska Institutet, Stockholm SE-171 77, Sweden.
4. Molecular Neurobiology Laboratory, The Salk Institute, La Jolla, CA 92186, USA.