1. Marine Biological Laboratory, Woods Hole, Massachusetts 02543, Center for Hemostasis and Thrombosis Research, Division of Hematology−Oncology, New England Medical Center, and Departments of Medicine and Biochemistry, Tufts University School of Medicine, Boston, Massachusetts 02111, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709, and Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599