1. Sections of *Cell and Developmental Biology and
2. Molecular Biology, Division of Biological Sciences, University of California-San Diego Medical School, La Jolla, CA 92093-0347;
3. Laboratory of Gene Regulation and Development, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892-5431; and
4. Department of Cell Biology, University of Texas Southwestern Medical Center, Dallas, TX 75390-9039