1. Cambridge Centre for Molecular Recognition, Department of Biochemistry, University of Cambridge; Tennis Court Road Cambridge CB2 1QW UK
2. Du Pont Merck Pharmaceutical Company, Box 80328; Wilmington DE 19880-0328 USA
3. Centre for Protein Engineering, MRC Centre; Hills Road Cambridge CB2 2QH UK
4. The Babraham Institute, Babraham Hall, Babraham; Cambridge CB2 4AT UK