1. Department of Cancer Biology, Vanderbilt University Medical Center, Nashville, Tennessee.
2. Current address: Ventana Medical Systems, Tucson, Arizona.
3. Current address: Department of Pharmacological Sciences, State University of New York, Stony Brook, New York.
4. Department of Cell Biology, University of Alabama, Birmingham, Alabama.
5. Current address: Division of Biomedical Sciences, Imperial College London, London, United Kingdom.