1. Laborarory Animal Resource Center, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8575, Japan
2. Present address: Research Center for Animal Life Science, Shiga University of Medical Science, Seta, Ootsu, Siga 520-2192, Japan
3. Experimental Animal Division, RIKEN BioResource Center, 3-1-1 Koyadai, Tsukuba 305-0074, Japan