1. Department of Cell Biology, Georgetown University Medical Center, Washington, DC 20007, USA.
2. Department of Obstetrics, Gynecology, and Reproductive Sciences, University of California, San Francisco, CA 94143, USA.
3. Department of Cell Biology, University of Virginia School of Medicine, Charlottesville, VA 22908, USA.
4. Department of Biochemistry, Molecular Biology, and Cell Biology, Northwestern University, Chicago, IL 60208, USA.