1. Stanford Genome Technology Center, Stanford University, Palo Alto, CA 94304, 2Department of Biochemistry, 3Department of Statistics, 4Division of Infectious Diseases, Department of Medicine, Stanford University, Stanford, CA 94305, 5James J. Peters VA Medical Center, Bronx, NY 10468, 6VA Palo Alto Health Care System, Palo Alto, CA 94304, and 7Division of Oncology, Department of Medicine, Stanford University School of Medicine, Stanford, CA 94305, USA