1. Max-Planck Institute for Biophysical Chemistry, Department of Molecular Cell Biology, Am Fassberg, 37077 Göttingen, Germany
2. Hagedorn Research Institute, Department of Developmental Biology, Niels Steensensvej 6, DK-2820 Gentofte, Denmark
3. DIBIT, San Raffaele Scientific Institute, Via Olgenittina 58, I-20132 Milano,Italy
4. Centre de Biologie du Développement, UMR-5547 CNRS-Université P. Sabatier, 118 Route de Narbonne, F-31062 Toulouse, Cedex 04, France