1. Department of Neuropharmacology, The Scripps Research Institute, La Jolla, California 920371;
2. Molecular Virology Laboratory, Hospital “Ramon y Cajal,” Instituto Nacional de la Salud, Madrid 28034, Spain2;
3. Division of Viral and Rickettsial Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia 303333; and
4. Department of Medicine, University of California, Irvine, College of Medicine, Orange, California 928684