1. Cambridge Institute for Medical Research, University of Cambridge, Hills Road, Cambridge CB2 0XY, UK.
2. Department of Medicine, University of Cambridge Clinical School, Addenbrookes Hospital, Hills Road, Cambridge CB2 2QQ, UK.
3. Paterson Institute for Cancer Research, University of Manchester, Wilmslow Road, Withington, Manchester M20 4BX, UK.
4. Laboratory for Molecular Biology, Children's Hospital, Charite Universitatsmedizin Berlin, Ziegelstrasse 5-9, D-10117 Berlin, Germany.
5. Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115, USA.