1. Department of Veterinary Surgery, College of Veterinary Medicine, Seoul National University, Seoul 151-742, Korea.
2. Laboratory of Stem Cell and Tumor Biology, Department of Veterinary Public Health, College of Veterinary Medicine, Seoul National University, Seoul 151-742, Korea.
3. College of Veterinary Medicine, Research Institute of Veterinary Medicine, Chungnam National University, Daejeon 305-764, Korea.