1. Thrombosis and Haemostasis Laboratory, Department of Haematology, University Medical Centre, P.O. Box 85500, 3508 GA Utrecht, The Netherlands, Institute for Biomembranes, University of Utrecht, Padualaan 8, 3584 CA Utrecht, The Netherlands, Department of Vascular Medicine, Academic Medical Centre, P.O. Box 22660, 1100 DD Amsterdam, The Netherlands, and Department of Cell Biology and Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, USA