1. From PRESTO, Japan Science and Technology Corporation, and Tsukuba Life Science Center, The Institute of Physical and Chemical Research (RIKEN), Tsukuba, Ibaraki 305, Japan; the Institute of Molecular Embryology and Genetics, Kumamoto University School of Medicine, Kumamoto 862, Japan; the Division of Radiation Health, National Institute of Radiological Sciences, Inage-ku, Chiba 263, Japan; and t