1. EMBL-European Bioinformatics Institute, Wellcome Trust Genome Campus, Hinxton, Cambridge CB10 1SD, UK, 2Sanger Institute, Wellcome Trust Genome Campus, Hinxton, Cambridge CB10 1SA, UK, 3Department of Physics, Sapienza University of Rome, P.le A. Moro 2, 00185 Rome, Italy, 4INFN, Sezione di Roma1, P.le A. Moro 5, 00185 Rome, Italy and 5HHMI Janelia Farm Research Campus, 19700 Helix Drive, Ashburn, VA 20147, USA