1. Department of Biochemistry and Biophysics; University of San Francisco; San Francisco CA 94158-2517 USA
2. Department of Bioengineering; Stanford University; Stanford CA 94305 USA
3. Howard Hughes Medical Institute; Chevy Chase MD 20815 USA
4. Department of Genetics; Stanford University; Stanford CA 94305 USA
5. ChEM-H; Stanford University; Stanford CA 94305 USA