1. C. N. Coleman and H. B. Stone are at the Radiation Research Program, Division of Cancer Treatment & Diagnosis, National Cancer Institute, Bethesda, MD 20892, USA.
2. J. Moulder is at the Medical College of Wisconsin, Milwaukee, WI 53226, USA.
3. T. C. Pellmar is at the Armed Forces Radiobiology Research Institute, Uniformed Services University of the Health Sciences, Bethesda, MD 20889, USA.