1. Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States
2. Department of Pathology, Harvard Medical School, Boston, Massachusetts 02115, United States
3. Department of Cancer Biology, Dana-Farber Cancer Institute, 44 Binney Street, Smith 756, Boston, Massachusetts 02115, United States
4. Mass Spectrometry and Proteomics Resource Laboratory, Center for Systems Biology, Harvard University, Cambridge, Massachusetts 02138, United States