1. Whitehead Institute, Nine Cambridge Center, Cambridge, MA 02142, and Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139.
2. Department of Pathology, University of Wash4on, Seattle, WA 98195.
3. Department of Biology, McGill University, Montreal, Quebec, Canada 13A 1B1.
4. MRC Mammalian Development Unit, Wolfson House, University College London, London, United Kingdom NW1 2HE.