1. Department of Chemistry and BiochemistryUniversity of California, Los Angeles Los Angeles CA 90095 USA
2. Pasarow Mass Spectrometry LaboratoryJane and Terry Semel Institute for Neuroscience and Human BehaviorDavid Geffen School of MedicineUniversity of California, Los Angeles Los Angeles CA 90095 USA
3. Department of Radiation OncologyDavid Geffen School of MedicineUniversity of California, Los Angeles Los Angeles CA 90095 USA
4. Department of Biological ChemistryDavid Geffen School of MedicineMolecular Biology Institute, and UCLA/DOE Institute for Genomics and ProteomicsUniversity of California, Los Angeles Los Angeles CA 90095 USA