1. Center for Virology and Vaccine Research, Beth Israel Deaconess Medical Center, Boston, Massachusetts 02215, USA;
2. Ragon Institute of Massachusetts General Hospital, MIT, and Harvard, Boston, Massachusetts 02114, USA
3. Theoretical Biology and Biophysics Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
4. New Mexico Consortium, Los Alamos, New Mexico 87545, USA