1. Cell and Developmental Biology Program at Harvard Medical School, Boston, Massachusetts 02115.
2. Dana-Farber Cancer Institute, Boston, Massachusetts 02115.
3. Department of Pathology, Harvard Medical School, Boston, Massachusetts 02115.
4. Department of Medicine, Tufts University School of Medicine, Boston, Massachusetts 02111.
5. Department of Biological Chemistry and Committee on Cell and Developmental Biology, Harvard Medical School, Boston, Massachusetts 02115.