1. Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115
2. Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, People's Republic of China
3. Laboratory of Computational Genomics, The Rockefeller University, New York, New York 10021
4. Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92037