1. MRC Centre for Regenerative Medicine, Institute for Stem Cell Research, School of Biological Sciences, University of Edinburgh, Edinburgh EH16 4UU, Scotland, UK
2. European Molecular Biology Laboratory, Mouse Biology Unit, Monterotondo Scalo 00015, Italy
3. Weatherall Institute of Molecular Medicine, University of Oxford, John Radcliffe Hospital, Headington, Oxford OX3 9DS, England, UK