1. Section on Cellular Neurobiology, Laboratory of Developmental Neurobiology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892, Novartis Institute for Medical Sciences, London WC1E 6BN, U.K., Center for Molecular Modeling, Center for Information Technology, National Institutes of Health, Bethesda, Maryland 20892, and Department of Biochemistry and Cell Biology, State University of New York, Stony Brook, New York 11794