1. Université Libre de Bruxelles, Laboratory of Stem Cells and Cancer, Brussels B-1070, Belgium.
2. Department of Haematology, Cambridge Institute for Medical Research, University of Cambridge, Cambridge CB2 0XY, UK.
3. Wellcome and Medical Research Council Cambridge Stem Cell Institute, University of Cambridge, Cambridge, UK.
4. WELBIO, Université Libre de Bruxelles, Brussels B-1070, Belgium.