1. Department of Cell Biology and Physiology, Box 8228, 660 South Euclid Avenue, St. Louis, MO 63110, U.S.A.
2. Department of Medicine, Washington University School of Medicine, Box 8228, 660 South Euclid Avenue, St. Louis, MO 63110, U.S.A.
3. Department of Anatomy and Histology, School of Dental Medicine, University of Pennsylvania, 4001 Spruce Street, Philadelphia, PA 19104, U.S.A.