1. Leiden Institute of Chemistry, Gorlaeus Laboratories, P.O. Box 9502, 2300 RA Leiden, The Netherlands, National Institute of Public Health and the Environment, Research Laboratory for Infectious Diseases, P.O. Box 1, 3720 BA Bilthoven, The Netherlands, and TNO, Prins Maurits Laboratory, P.O. Box 45, 2280 AA Rijswijk, The Netherlands