1. Departments of Developmental Biology, Genetics, and Bioengineering, Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, California 94305
2. Howard Hughes Medical Institute, Department of Molecular and Human Genetics, Stanford University School of Medicine, Stanford, California 94305
3. Department of Physiology Green Center Division for Systems Biology, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9040
4. Program in Developmental Biology, Baylor College of Medicine, Houston, Texas 77030
5. Department of Genome Biology, Lawrence Berkeley National Laboratory, Berkeley, California 94720-3200 and