Protein characterization. iTRAQ-TAILS Validation by Glu-C Cleavage of Native Proteins--For iTRAQ-TAILS validation, we used a mixture of 13 chemokines of which one was a synthetic analog of MMPcleaved CXC motif chemokine 8 (from position 5 to 72 (5?2)) of the full-length (1?2) chemokine (supplemental Table 1). To check the accuracy of neo-N-terminal peptide identification, the chemokine mixture was