1. Maxygen, 515 Galveston Drive, Redwood City, California 94063;
2. Department of Molecular Pharmacology and Biological Chemistry, Northwestern University Medical School, Chicago, Illinois 60611;
3. Department of Biochemistry and Biophysics, University of California, San Francisco, California 94143;
4. The Howard Hughes Medical Institute, University of California, San Francisco, California 94143;