1. Department of Medicine, University College London, 5 University Street, London, W1P 8BT, U.K.
2. MRC-LMCB, University College London, Gower Street, London, WC1E 6BT, U.K.
3. Ludwig Institute for Cancer Research, 91 Riding House Street, London, W1P 8BT, U.K.
4. Department of Pharmaceutical Chemistry, The Mass Spectrometry Facility and Liver Center, University of California, San Francisco, CA, U.S.A., and Department of Biochemistry and Molecular Biology
5. University College London, Gower Street, London, WC1E 6BT, U.K.