1. Department of Chemistry, Imperial College London, London W12 0BZ, U.K.
2. National Heart and Lung Institute, Imperial College London, London SW7 2AZ, U.K.
3. Division of Structural Biology, University of Oxford, Oxford OX3 7BN, U.K.
4. Division of Cancer Therapeutics, Centre for Cancer Drug Discovery, Institute of Cancer Research, London SM2 5NG, U.K.