1. Department of Molecular and Human Genetics, Howard Hughes Medical Institute, Program in Developmental Biology, Baylor College of Medicine, Houston, Texas 77030
2. Department of Embryology, Howard Hughes Medical Institute Research Laboratories, Carnegie Institution of Washington, Baltimore, Maryland 21210
3. Department of Molecular and Cellular Biology, Howard Hughes Medical Institute, University of California, Berkeley, California 94720-3200
4. Lawrence Berkeley National Laboratory, Berkeley, California 94720-3200