1. Platform for Molecular Biology and Biotechnology, Scientific Institute of Public Health, J. Wytsmanstreet 14, B-1050 Brussels, 2 Department of Applied Mathematics and Computer Science, Ghent University, Krijgslaan 281, S9 B-9000 Gent, Belgium and 3 European Commission, Joint Research Center, Institute for Health and Consumer Protection, Molecular Biology and Genomics Unit, via E. Fermi 2749, 21027 Ispra (VA), Italy