1. Bioinformatics Institute Ghent and Department of Applied Mathematics, Computer Science and Statistics, Ghent University, 9000 Ghent, Belgium
2. Institute of Molecular Life Sciences and SIB Swiss Institute of Bioinformatics, University of Zurich, 8057 Zurich, Switzerland;
3. Department of Biostatistics and Department of Genetics, University of North Carolina, Chapel Hill, North Carolina 27514, USA
4. Department of Computer Science, Stony Brook University, Stony Brook, New York 11794, USA