1. Department of Cell Biology, Harvard Medical School, Boston 02115, Massachusetts, United States
2. Department of Cancer Biology, Dana-Farber Cancer Institute, Boston 02215, Massachusetts, United States
3. Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston 02115, Massachusetts, United States
4. Thermo Fisher Scientific, Rockford 61101-9316, Illinois, United States
5. Proteome Sciences, London WC1H 9BB, U.K.