1. Ludwig Institute for Cancer Research, 91 Riding House Street, London W1W 7BS, UK.
2. Gene Targeting Laboratory, Center for Genome Research, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JQ, UK.
3. Frimorfo, Rue du Musée 12, CH-1705 Fribourg, Switzerland.
4. Department of Biochemistry and Molecular Biology, University College London, Gower Street, London WC1E 6BT, UK.