1. L. Zheng, R. Wang, H. Erfle, H. Voss, W. Ansorge, F. C. Kafatos, European Molecular Biology Laboratory, Meyerhofstrasse 1, 69117 Heidelberg, Germany.
2. A. J. Cornel and F. H. Collins, Centers for Disease Control and Prevention, 4770 Buford Highway, Mail Stop F22, Chamblee, GA 30341, USA.