1. Institut Jacques Monod, Centre National de la Recherche Scientifique-UMRC7592, Université Paris 7-Denis Diderot, Paris Cedex 05, France;
2. VTT Biotechnology and Food Research, FIN-02044 VTT, Finland;
3. Biozentrum, University of Basel, CH-4056 Basel, Switzerland; and
4. Department of Molecular Genetics and Cell Biology, The University of Chicago, Chicago, Illinois 60637