1. E. C. Butcher is in the Laboratory of Immunology and Vascular Biology, Department of Pathology, Stanford University School of Medicine, Stanford, CA 94305- 5324, USA, and the Center for Molecular Biology and Medicine, the Veterans Affairs Palo Alto Health Care System, Palo Alto, CA 94304, USA.
2. L. J. Picker is in the Laboratory of Molecular Pathology, Department of Pathology, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75235- 9072, USA.