1. Department of Medicine, Tufts University School of Medicine, Boston, Massachusetts 02111
2. Department of Surgery, New England Medical Center Hospital, Boston, Massachusetts 02111
3. Department of Biochemistry, Downstate Medical School, Brooklyn, New York 11203
4. Departments of Medicine and Physiology, Tufts University Schools of Medicine, Dental Medicine, and Veterinary Medicine, and Sackler School of Graduate Biomedical Sciences, Boston 02111