1. Faculty of Biology, Institute of Biology III, University of Freiburg, Schänzlestraße 18, 79104 Freiburg, Germany
2. Signalling Research Centres BIOSS and CIBSS, University of Freiburg, 79104 Freiburg, Germany
3. European Molecular Biology Laboratory, Genome Biology Unit, Meyerhofstraße 1, 69117 Heidelberg, Germany