1. Advanced BioScience Laboratories–Basic Research Program, National Cancer Institute–Frederick Cancer Research and Development Center, Frederick, Maryland 21702
2. Hoffmann-LaRoche, Nutley, New Jersey 07110
3. Science Applications International Corporation, National Cancer Institute–Frederick Cancer Research and Development Center, Frederick, Maryland 21702
4. Department of Cell Biology and Anatomy, University of Calgary, Faculty of Medicine, Calgary, Canada T2N 4N1