1. Department of Molecular Genetics and Biotechnology, The Hebrew University-Hadassah Medical School, Jerusalem 91120
2. Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot, Israel 76100
3. Gene Regulation and Chromosome Biology Laboratory, National Cancer Institute at Frederick, Frederick, Maryland 21702
4. Laboratory of Molecular Biology, National Cancer Institute, Bethesda, Maryland 20892-4264;