1. Department of Genetics and Development, Columbia University Medical Center, New York, New York 10032
2. Swiss Institute for Experimental Cancer Research, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
3. National Centre of Competence in Research “Frontiers in Genetics,” CH-1211 Geneva, Switzerland
4. Institute of Biochemistry, Swiss Federal Institute of Technology, CH-8093 Zürich, Switzerland and
5. Banting and Best Department of Medical Research and Department of Molecular Genetics, Terrence Donnelly Centre for Cellular and Biomolecular Research, University of Toronto, Toronto, Ontario M5S 3E1, Canada