1. Friedrich Miescher Institute for Biomedical Research, Maulbeerstrasse 66, 4058 Basel, Switzerland.
2. Department of Molecular Biology and National Center of Competence in Research Frontiers in Genetics, 30 Quai Ernest Ansermet, 1211 Geneva, Switzerland.
3. Department of Biochemistry, Donnelly Centre for Cellular and Biomolecular Research, University of Toronto, 160 College Street, Toronto, Ontario M5S 3E1, Canada.
4. Department of Cellular and Molecular Pharmacology, California Institute for Quantitative Biomedical Research, University of California, San Francisco, 1700 Fourth Street, San Francisco, CA 94158, USA.