1. Department of Biology, McGill University, Montréal, Québec, Canada
2. Center for Cancer Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
3. Institute for Molecular Cell Biology, BioCentrum Amsterdam, 1098 SM Amsterdam, The Netherlands
4. Department of Biology, Yale University, New Haven, Connecticut 06520
5. Department of Microbiology and Immunology, University of British Columbia, Vancouver, British Columbia, Canada