1. Department of Genetic Medicine, Weill Medical College of Cornell University, 1300 York Avenue, New York, NY 10021, USA.
2. Laboratory of Chromatin Biology, Rockefeller University, Box 78, 1230 York Avenue, New York, NY 10021, USA.
3. Department of Chemistry and Biochemistry and Molecular Biology Institute, University of California at Los Angeles, Los Angeles, CA 90095–1569, USA.
4. Department of Pathology, University of Southern California, Los Angeles, CA 90089–9092, USA.
5. Department of Microbiology and Immunology, Baylor College of Medicine, Houston, TX 77030, USA.