1. Division of Genetic Resources, National Institute of Health Japan, Toyama, Shinjukuku, Tokyo 162, Japan
2. Department of Radiology, The Jikei University School of Medicine, Nishishinbashi, Minatoku, Tokyo 105, Japan
3. Laboratory of Cell Growth and Differentiation, Tsukuba Life Science Centre, The Institute of Physical and Chemical Research (RIKEN), Koyadai, Tsukuba, Ibaraki 305, Japan