Fondazione Bruno Kessler, I-38100 Povo (Trento), Italy, European Molecular Biology Laboratory, European Bioinformatics Institute, Cambridge CB10 1SD, UK, Wellcome Trust Sanger Institute, Cambridge CB10 1SD, UK and Universitat Pompeu Fabra, Barcelona 08003, Spain Fondazione Bruno Kessler, I-38100 Povo (Trento), Italy, European Molecular Biology Laboratory, European Bioinformatics Institute, Cambridge CB10 1SD, UK, Wellcome Trust Sanger Institute, Cambridge CB10 1SD, UK and Universitat Pompeu Fabra, Barcelona 08003, Spain.