1. Graduate School of Agricultural Science; Tohoku University; 1-1 Tsutsumidori-amamiyamachi Aoba-ku Sendai 981-8555 Japan
2. Division of Virology; National Cancer Center Research Institute; 5-1-1 Tsukiji Chuo-ku Tokyo 104-0045 Japan
3. Faculty of Applied Life Sciences; Niigata University of Pharmacy and Applied Life Sciences; 265-1 Higashijima Akiha-ku Niigata 956-8603 Japan
4. Animal Research Center; Sapporo Medical University; S1 W17 Chuo-ku Sapporo 060-8556 Japan