1. Mammalian Development Laboratory, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK
2. Wellcome Trust/Cancer Research UK Institute, University of Cambridge, Tennis Court Road, Cambridge CB2 1QR, UK
3. Department of Developmental Biology, MaxPlanck Institut of Immunobiology, Stubeweg 51, D-79011 Freiburg, Germany