1. Division of Cancer Epidemiology and Genetics National Cancer Institute Bethesda, Maryland 20892 U.S.A.
2. University of Maryland College Park, Maryland 20742 U.S.A.
3. Johns Hopkins Bloomberg School of Public Health Baltimore, Maryland 21205 U.S.A.
4. Dana Farber Cancer Institute and Harvard School of Public Health Boston, Massachusetts 02115 U.S.A.