1. Istituto Nazionale per la Ricerca sul Cancro (IST), 16132 Genova, Italy
2. Department of Biochemistry & Molecular Genetics, University of Alabama at Birmingham, Birmingham, AL 35294, USA
3. Howard Hughes Medical Institute, Department and School of Medicine, University of California, San Diego, 9500 Gilman Drive, Room 345, La Jolla, CA 92093‐0648, USA
4. Division of Molecular Structure, MRC National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK