1. Department of Cell and Developmental Biology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232
2. Department of Genetics, Evolution and Environment and UCL Cancer Institute, University College London, London WC1E 6BT, United Kingdom
3. Genome Technology Center, Stanford University School of Medicine, Palo Alto, California 01605
4. Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, Massachusetts 01605