Dorit B. Donoviel, Anna-Katerina Hadjantonakis, Masaki Ikeda, Hui Zheng, Peter St George Hyslop, and Alan Bernstein Samuel Lunenfeld Research Institute, Mount Sinai Hospital, Toronto, Ontario, Canada M5G-1X5; Centre for Research in Neurodegenerative Diseases and Department of Medicine, Division of Neurology, University of Toronto. The Toronto Hospital, Toronto, Ontario, Canada M5S-3H2; Merck Research Laboratories, Rahway, New Jersey 07065 USA; Department of Medical Genetics and Microbiology, University of Toronto, Toronto, Ontario, Canada M5S-1A8

Mice lacking both presenilin genes exhibitearlyembryonic patterningdefects