1. Receptor Biology Section, National Institutes of Health, P.O. Box 12233, Research Triangle Park, NC 27709
2. Gamete Biology Section, Laboratory of Reproductive and Developmental Toxicology, National Institute of Environmental Health Sciences, National Institutes of Health, P.O. Box 12233, Research Triangle Park, NC 27709
3. Departments of Obstetrics & Gynecology and Cell Biology, Duke University Medical Center, Duke University, Durham, NC 27710, U.S.A.