1. Department of Biological Structure, University of Washington School of Medicine, Seattle, Washington 98195, U.S.A.
2. Departments of Biological Structure and Zoology, University of Washington School of Medicine, Seattle, Washington 98195, U.S.A.; Present address: Department of Anatomy, University of California Medical School, San Francisco, California, U.S.A.
3. Departments of Medicine and Obstetrics and Gynecology, University of Washington School of Medicine, Seattle, Washington 98195, U.S.A.; Investigator of the Howard Hughes Medical Institute