1. Division of Stem Cell Biology and Developmental Genetics, Medical Research Council National Institute for Medical Research, London NW7 1AA, England, UK
2. Institut National de la Santé et de la Recherche Medicale U741, Institut Jacques Monod, 75251 Paris, Cedex 05, France
3. Center for Reproductive Medicine, Academic Medical Center, University of Amsterdam, Amsterdam 1105 AZ, Netherlands
4. Department of Endocrinology and Metabolism, Faculty of Science, Utrecht University, Utrecht 3584 CH, Netherlands
5. Department of Genetics and Development, College of Physicians and Surgeons of Columbia University, New York, NY 10032