1. Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, 330 Brookline Avenue, Boston, MA 02215, USA.
2. Merck Research Laboratories, West Point, PA 19486, USA.
3. Southern Research Institute, 431 Aviation Way, Frederick, MD 21701, USA.
4. Duke University Medical Center, Durham, NC 27710, USA.
5. Department of Biostatistical Science, Dana-Farber Cancer Institute, 44 Binney Street, Boston, MA 02115, USA.