1. Department of Virology, Leiden University Medical Center, 2300 RC Leiden, The Netherlands1;
2. EMBL, 69117 Heidelberg, Germany2; and
3. Department of Pediatrics,3
4. Department of Microbiology and Immunology,4 and the
5. Elizabeth B. Lamb Center for Pediatric Research,5Vanderbilt University Medical Center, Nashville, Tennessee 37232