1. Howard Hughes Medical Institute, Chevy Chase, Maryland 20815
2. Department of Tumor Cell Biology, St. Jude Children's Research Hospital, Memphis, Tennessee 38105;
3. Department of Biochemistry and Biophysics, Division of Translational Medicine and Chemical Biology, Science for Life Laboratory, Karolinska Institute, Stockholm S-171 21, Sweden
4. Danish Cancer Society Research Center, Copenhagen DK 2100, Denmark