1. Center for Virology and Vaccine Research, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA 02215, USA.
2. Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
3. Gilead Sciences Inc., Foster City, CA 94404, USA.
4. Program for Evolutionary Dynamics, Harvard University, Cambridge, MA 02138, USA.
5. Bioqual, Rockville, MD 20852, USA.
6. Ragon Institute of Massachusetts General Hospital, Massachusetts Institute of Technology, and Harvard, Cambridge, MA 02139, USA.