1. Genome Sciences Department, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
2. Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA.
3. Departments of Medicine and Microbiology/Immunology and Howard Hughes Medical Institute, University of California, San Francisco, CA 94143, USA.
4. Department of Computer Science and Engineering, Pennsylvania State University, University Park, PA 16802, USA.