1 University of Alberta, Edmonton, AB T6G 2R3, Canada.
2 RIKEN Center for Developmental Biology, 2-2-3 Minatojima-minamimachi, Chuou-ku, Kobe 650-0047, Japan; Keio University School of Medicine, Shinjuku-ku, Tokyo 160-8582, Japan. Electronic address: sipp@cdb.riken.jp.