1. Hubrecht Institute-KNAW and University Medical Center, Uppsalalaan 8, 3584 CT, Utrecht, The Netherlands
2. Department of Pathobiology, Faculty of Veterinary Medicine, Utrecht University, PO Box 80.163, 3508 TD, Utrecht, The Netherlands
3. Institute of Biology, Leiden University, PO Box 9502, 2300 RA, Leiden, The Netherlands