1. The Salk Institute for Biological Studies, Laboratory of Genetics, La Jolla, CA 92037, USA.
2. National Institutes of Health, National Institute of Diabetes and Digestive Diseases, Bethesda, MD 20892, USA.
3. Center for Cancer Research, National Cancer Institute, Frederick, MD 21702, USA.
4. Department of Integrative Structural and Computational Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.