1. D. C. Thomis, C. B. Gurniak, L. J. Berg, Department of Molecular and Cellular Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA 02138, USA.
2. E. Tivol and A. H. Sharpe, Departments of Pathology, Brigham and Women's Hospital and Harvard Medical School, 221 Longwood Avenue, Boston, MA 02115, USA.