1. From the Leukocyte Adhesion Laboratory and the Lymphocyte Molecular Biology Laboratory, Imperial Cancer Research Fund, London WC2A 3PX, United Kingdom; the Center of Anatomy, Medical School of Hannover, D-30623 Hannover, Germany; the Department of Immunology, Medical Clinic, University Hospital Eppendorf, D-20246 Hamburg, Germany; and Charité Clinic, Humboldt University, D-10117 Berlin, Germany