1. L. Chong, B. van Steensel, D. Broccoli, J. Hanish, T. de Lange, Laboratory for Cell Biology and Genetics, Rockefeller University, 1230 York Avenue, New York, NY 10021, USA.
2. H. Erdjument-Bromage and P. Tempst, Molecular Biology Program, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10021, USA.