1. Department of Developmental Biology, Genentech, Inc., South San Francisco, CA 94080 and Department of Biochemistry and Biophysics, University of California at San Francisco, San Francisco, CA 94143.
2. Department of Molecular Biology, Genentech, Inc., South San Francisco, CA 94080.
3. Department of Biochemistry and Biophysics, Oregon State University, Corvallis, OR 97331.