1. Cancer Research UK, Cambridge Research Institute, Li Ka Shing Centre, Cambridge CB2 0RE, UK.
2. School of Biological and Biomedical Sciences and NorthEast England Stem Cell Institute, Durham University, South Road, Durham DH1 3LE, UK.
3. Wellcome Trust Centre for Stem Cell Research, University of Cambridge, Tennis Court Road, Cambridge CB2 1QR, UK.