1. B. Li, W. J. Fairbrother, J. A. Wells, B. C. Cunningham, Department of Protein Engineering, Genentech, 460 Point San Bruno Boulevard, South San Francisco, CA 94080, USA.
2. J. Y. K. Tom and D. Oare, Bioorganic Chemistry, Gen- entech, 460 Point San Bruno Boulevard, South San Francisco, CA 94080, USA.
3. R. Yen, Medicinal and Analytical Chemistry, Genentech, 460 Point San Bruno Boulevard, South San Francisco, CA 94080, USA.