1. Wellcome Trust Centre for Cell Biology, University of Edinburgh, Edinburgh EH9 3QR, United Kingdom
2. Genome Structure and Function Group, Developmental Therapeutics Branch, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, United States
3. Research and Development, Cellay Inc., Cambridge, Massachusetts 02139, United States
4. Laboratory of Cell Engineering, Department of Frontier Research, Kazusa DNA Research Institute, Kisazaru 292-0818, Japan
5. Josep Carreras Leukaemia Research Institute, School of Medicine, University of Barcelona, Casanova 143, 08036 Barcelona, Spain