1. Department of Environmental and Radiological Health Sciences, Colorado State University, Fort Collins, Colorado 80523;
2. Department of Radiation Oncology, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75390;
3. Tohoku Medical and Pharmaceutical University, Sendai, Japan, 981-8558; and
4. Department of Genetics and Complex Diseases, Harvard School of Public Health, Boston, Massachusetts 02115