Center for Bioinformatics, Saarland University, Postfach 15 11 50, 66041 Saarbrücken, Germany. volkhard.helms@bioinformatik.uni-saarland.de.|Center for Bioinformatics, Saarland University, Postfach 15 11 50, 66041 Saarbrücken, Germany. volkhard.helms@bioinformatik.uni-saarland.de.