1. Department of Biological Sciences, Columbia University, 1212 Amsterdam Avenue, New York, New York 10027
2. Molecular Biology Program, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, New York 10021
3. Department of Molecular Biology, University of Copenhagen, DK-2200 Copenhagen N, Denmark
4. Department of Genetics and Development, Columbia University Medical Center, 701 West 168th Street, New York, New York 10032-2704