1. Henry Wellcome Building for Molecular Physiology, University of Oxford, Roosevelt Drive, Oxford OX3 7BN, U.K.
2. Howard Hughes Medical Institute, University of California, San Francisco, GH-N312E, MC 2200, 600 16th Street, San Francisco, California 94158-2517
3. Weatherall Institute of Molecular Medicine, John Radcliffe Hospital, Headington, University of Oxford, Oxford OX3 9DS, U.K.
4. Division of Structural Biology, Henry Wellcome Building for Genomic Medicine, Roosevelt Drive, Oxford OX3 7BN, U.K.