1. Laboratory of Biochemistry, B.C.P. Jansen Institute, University of Amsterdam, PO Box 20151, 1000 HD Amsterdam, The Netherlands
2. Laboratory of Cell Biology, The Rockefeller University, New York, NY 10021, U.S.A.
3. Developmental and Metabolic Neurology Branch, National Institute of Neurological and Communicative Disorders and Stroke, National Institutes of Health, Bethesda, MD 20205, U.S.A.