Author information:
(1)Laboratory of Viral Diseases, National Institute of Allergy and Infectious 
Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
(2)Laboratory of Viral Diseases, National Institute of Allergy and Infectious 
Diseases, National Institutes of Health, Bethesda, MD 20892, USA. Electronic 
address: piersontc@mail.nih.gov.