1. Computational Biology and Bioinformatics, Université Libre de Bruxelles , 1050 Brussels, Belgium
2. Interuniversity Institute of Bioinformatics in Brussels , 1050 Brussels, Belgium
3. John von Neumann Institute for Computing, Jülich Supercomputer Centre , 52428 Jülich, Germany
4. Department of Biology, University of Duisburg-Essen , D-45141 Essen, Germany