1. Section on Cellular Neurobiology, Laboratory of Developmental Neurobiology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892, Biochemistry Section, Surgical Neurology Branch, NINDS/NIGMS, National Institutes of Health, Bethesda, Maryland 20892, and Medvir UK Ltd., 100 Fulbourn Road, Cambridge CB1 9PT, United Kingdom