1. From the Department of Epidemiology and Biostatistics, School of Public Health and Health Services, George Washington University, Washington, D. C., Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, Maryland, Shanghai Cancer Institute, Shanghai, China, and Departments of Obstetrics and Gynecology and of Preventive Medicine, Keck School of Medicine, University of Southern California, Los Angeles, California