1. Department of Pediatrics, School of Medicine, Stanford University, Stanford, CA 94305.
2. Present address: Department of Virology, Max von Pettenkofer Institute, Ludwig Maximilians University, Munich 80336, Germany.
3. Department of Biochemistry and Molecular Genetics, Schools of Medicine and Dentistry, University of Alabama at Birmingham, Birmingham, AL 35294.
4. Department of Pathology, School of Medicine, Stanford University, Stanford, CA 94305.
5. Department of Genetics, School of Medicine, Stanford University, Stanford, CA 94305.