1. Department of Cell Biology, Graduate School of Medicine, Tohoku University, Sendai 980-8575, Japan
2. Radiation Effect Mechanisms Research Group, National Institute of Radiological Sciences, Chiba 263-8555, Japan
3. Department of Radiobiology, Institute for Environmental Sciences, Aomori 039-3212, Japan
4. Department of Radiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois 60611