1. J. A. Thomson, M. A. Waknitz, J. J. Swiergiel, V. S. Marshall, Wisconsin Regional Primate Research Center, University of Wisconsin, Madison, WI 53715, USA. J. Itskovitz-Eldor, Department of Obstetrics and Gynecology, Rambam Medical Center, Faculty of Medicine, Technion, Haifa 31096, Israel. S. S. Shapiro and J. M. Jones, Department of Obstetrics and Gynecology, University of Wisconsin, Madison, WI 53715, USA.