[1] Dept. Microbiol. and Molec. Genet., Harvard Med. Sch. Div. Cancer Biol., Boston, MA 02115, United States
[2] Department of Pathology, Harvard Medical School, Dana-Farber Cancer Institute, Boston, MA 02115, United States
[3] DuPont Pharmaceut Co. Exp. Stn. E., Wilmington, DE 19880, United States
[4] Dept. Microbiol. and Molec. Genet., Dana-Farber Cancer Inst., 44 Binney St., Boston, MA 02115, United States