1. From the Division of Blood Diseases and Resources and the Biometrics Branch, National Heart, Lung, and Blood Institute, National Institutes of Health, the National Hemophilia Foundation, the University Hospital, Utrecht, Netherlands, the University of Connecticut, Farmington, CT, and the Scripps Research Institute, La Jolla, CA (address reprint requests to Dr. Klein at the Clinical Center Blood Bank, Bldg. 10A, Rm. 1E33, National Institutes of Health, Bethesda, MD 20014).