1. Viral Mutation Section, HIV Drug Resistance Program, National Cancer Institute at Frederick, Frederick, MD 21702, USA.
2. Department of Molecular Biology and Microbiology, School of Medicine, Tufts University, 150 Harrison Avenue, Boston, MA 02111, USA.
3. Genetics Program, School of Medicine, Tufts University, 150 Harrison Avenue, Boston, MA 02111, USA.
4. Department of Biochemistry and Molecular Medicine, University of California, Davis, Sacramento, CA 95817, USA.
5. Department of Urology, University of California, Davis, Sacramento, CA 95817, USA.
6. Viral Recombination Section, HIV Drug Resistance Program, National Cancer Institute at Frederick, Frederick, MD 21702, USA.
7. Data Management Services, Inc., National Cancer Institute at Frederick, Frederick, MD 21702, USA.