1. Department of Genetics, University of Cambridge; Cambridge UK
2. Unité de Génétique et Biologie du Développement, Institut Curie; Paris France
3. Institut für Genetik, Heinrich-Heine-Universität; Düsseldorf Germany
4. Department of Physics, Cavendish Laboratory, University of Cambridge; Cambridge UK
5. The Wellcome Trust/Cancer Research UK Gurdon Institute, University of Cambridge; Cambridge UK