1. Department of Biochemistry, Dartmouth Medical School, Hanover, New Hampshire 03755
2. Departments of Cell-Developmental Biology, Obstetrics-Gynecology, and Oregon Regional Primate Research Center, Oregon Health Sciences University, Beaverton, Oregon 97006
3. Rippel Electron Microscope Facility, Dartmouth College, Hanover, New Hampshire 03755
4. Department of Genetic Information, Division of Molecular Life Science, University School of Medicine, Bohseidai, Isehara, Kanagawa 259-1193, Japan