1. Institute for Genetics, University of Köln Weyertal 121 50931 Köln Germany
2. Department of Cell Biology, Gesellschaft für Biotechnologische Forschung (GBF) Mascheroder Weg 1 38124 Braunschweig Germany
3. Institute of Anatomy, University of Bonn Nussallee 10 53115 Bonn Germany
4. Present address: Artemis Pharmaceuticals GmbH Neurather Ring 1 51063 Köln Germany