1. S. Tanaka and A.-K. Hadjantonakis, Samuel Lunenfeld Research Institute, Mount Sinai Hospital, Toronto, Ontario M5G 1X5, Canada. T. Kunath, A. Nagy, J. Rossant, Samuel Lunenfeld Research Institute, Mount Sinai Hospital, Toronto, Ontario M5G 1X5, Canada, and Department of Molecular and Medical Genetics, University of Toronto, Toronto, Ontario M5S 1A8, Canada.