1. Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720, USA
2. Medical Research Council Human Genetics Unit, Institute of Genetics and Molecular Medicine, University of Edinburgh, Edinburgh EH4 2XU, UK
3. European Molecular Biology Laboratory (EMBL), Genome Biology Unit, 69117 Heidelberg, Germany
4. Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco, CA 94158, USA