1. Division of Biology, California Institute of Technology, Pasadena, CA 91125, USA.
2. Howard Hughes Medical Institute, California Institute of Technology, Pasadena, CA 91125, USA.
3. Department of Computational, Analytical, and Structural Sciences, GlaxoSmithKline, King of Prussia, PA 19406, USA.
4. Research Institute of Molecular Pathology, Dr. Bohr-Gasse 7, A-1030, Vienna, Austria.