1. M. Emerman is in the Division of Molecular Medicine, Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North–Mailstop C2–023, Seattle, Washington 98109–1024, USA.. M. H. Malim is in the Department of Microbiology, Howard Hughes Medical Institute, University of Pennsylvania, 415 Curie Boulevard, Room 347B, Philadelphia, PA 19104–6148, USA.