1. Institute of Cell Biology, Department of Biology, Swiss Federal Institute of Technology, ETH-Hönggerberg, CH-8093 Zurich, Switzerland.
2. Department of Molecular Embryology, Max-Planck Institute of Immunobiology, Stuebeweg 51, D-79108 Freiburg, Germany.
3. Department of Pharmacology, Graduate School of Medicine, Kyoto University, Yoshida-Konoé-cho, Sakyo, Kyoto 606-8501, Japan.