1. C. R. Beals and G. R. Crabtree, Howard Hughes Medical Institute, Department of Developmental Biology, Stanford University, Stanford, CA 94305, USA.
2. C. M. Sheridan and P. Gardner, Department of Molecular Pharmacology, Stanford University, Stanford, CA 94305, USA.
3. C. W. Turck, Howard Hughes Medical Institute, University of California, San Francisco, CA 94143, USA.