1. Department of Oral Biology, BK21 PLUS Project, Yonsei University College of Dentistry, Seoul 120-752, Korea.
2. Department of Pediatric Dentistry, Yonsei University College of Dentistry, Seoul 120-752, Korea.
3. Department of Oral Physiology, College of Dentistry, Wonkwang University, Iksan 570-749, Korea.