1. The Wellcome Trust/Cancer Research UK (CRUK) Gurdon Institute and Department of Biochemistry, University of Cambridge, CB2 1QN Cambridge, UK.
2. Institut de Chimie des Substances Naturelles, CNRS, 91198 Gif-sur-Yvette, France.
3. The Wellcome Trust Sanger Institute, Hinxton, CB10 1SA Cambridge, UK.