1. Laboratory of Veterinary Pathology; Osaka Prefecture University; Izumisano Osaka 598-8531 Japan
2. Wellcome Trust-Medical Research Council Stem Cell Institute and Department of Clinical Neurosciences; University of Cambridge, Addenbrookes Hospital; Cambridge CB2 0SP United Kingdom
3. Institute of Laboratory Animals, Graduate School of Medicine; Kyoto University; Sakyo-ku Kyoto 606-8501 Japan