1. Hubrecht Laboratory, Netherlands Institute for Developmental Biology, Uppsalalaan 8, 3584 CT Utrecht, The Netherlands
2. Present address: Institute of Hematology, Erasmus University, Dr Molewaterplein 50, 3015 GR, Rotterdam, The Netherlands
3. Howard Florey Institute of Experimental Physiology and Medicine, University of Melbourne, Australia
4. Department of Biochemistry, University of Leicester, Leicester LE1 7RH, UK
5. Department of Molecular Cell Biology, Max Planck Institute for Biophysical Chemistry, D-37077 Göttingen, Germany