1. F. H. Espinoza, A. Farrell, D. O. Morgan, Department of Physiology and Department of Biochemistry and Biophysics, University of California, San Francisco, CA 94143-0444, USA.
2. H. Erdjument-Bromage and P. Tempst, Molecular Biology Program, Memorial Sloan-Kettering Cancer Center, New York, NY 10021, USA.