1. Institute of Structural and Molecular Biology, Division of Biosciences, University College London, Gower Street, London WC1E 6BT, UK
2. Department of Physics, Imperial College London, South Kensington, London SW7 2AZ, UK
3. Division of Cancer Biology, Chester Beatty Laboratories, The Institute of Cancer Research, Fulham Road, London SW3 6JB, UK