1. Biomolecular Research Laboratory, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
2. Swiss Federal Institute of Technology Zürich, CH-8093 Zürich, Switzerland
3. Department of Evolutionary Biology, University of Siena, 53100 Siena, Italy
4. Department of Molecular, Cellular, and Developmental Biology, Yale University, New Haven, CT 06520