1. Department of Pathology, Brigham and Women's Hospital and Harvard Medical School, Boston, MA 02115, USA.
2. Authors are alphabetized to reflect equal contributions. Comments and ideas expressed herein are their own.
3. DNAVIEW, Oakland, CA 94611, and School of Public Health, University of California, Berkeley, CA 94720, USA.
4. John F. Kennedy School of Government, Harvard University, Cambridge, MA 02138, USA.