1,2,3 1Department of Microbiology, Immunology, and Molecular Genetics, University of California, Los Angeles, Los Angeles, California 90095, USA; 2Molecular Biology Institute, University of California, Los Angeles, Los Angeles, California 90095, USA; 3Howard Hughes Medical Institute, University of California, Los Angeles, Los Angeles, California 90095, USA Corresponding author: smale{at}mednet.ucla.edu Footnotes Supplemental material is available for this article.