1. Institut de Biologie de l’ENS (IBENS), Ecole Normale Supérieure, Université PSL, CNRS, INSERM, F-75005 Paris, France.
2. MRC Laboratory of Molecular Biology, Cambridge CB2 0QH, UK.
3. Division of Structural Biology, Wellcome Centre for Human Genetics, University of Oxford, Oxford OX3 7BN, UK.