1. European Bioinformatics Institute, EMBL, Wellcome Trust Genome Campus, Hinxton, Cambridge CB10 1SD, England, Department of Biological and Molecular Sciences, University of Stirling, Stirling FK9 4LA, Scotland, UCMB, Université Libre de Bruxelles, CP 160/16, Avenue P. Héger, 1050-Bruxelles, Belgium, and Laboratoire d'Enzymologie et de Biochimie Structurales, CNRS UPR9063, 91198-Gif-sur-Yvette, France