1. Department of Molecular Genetics, Hoffmann-La Roche, Inc., Roche Research Center, Nutley, NJ 07110.
2. Department of Molecular Oncology, Hoffinann-La Roche, Inc., Roche Research Center, Nutley, NJ 07110.
3. Laboratory of Tumor Cell Biology, National Cancer Institute, Bethesda, MD 20205.