1. Experimental Animal Division, RIKEN BioResource Center, Tsukuba, Ibaraki 305-0074, Japan
2. Microbe Division/Japan Collection of Microorganisms, RIKEN BioResource Center, Tsukuba, Ibaraki 305-0074, Japan
3. Center for Human Resources Development, National Institute of Radiological Sciences, Chiba 263-8555, Japan
4. Research, Development and Support Center, National Institute of Radiological Sciences, Chiba 263-8555, Japan