1. Centre for Molecular Cell Biology, Department of Medicine, Royal Free and University College Medical School, University College London, Rowland Hill Street, London NW3 2PF, U.K.
2. Mass Spectrometry & Bioanalytical Chemistry Laboratory, Ludwig Institute for Cancer Research, Cruciform Building, Gower Street, London WC1E 6BT, U.K.
3. Department of Biochemistry and Molecular Biology, University College London, Gower Street, London WC1E 6BT, UK