Fig. (4) Example of MS/MS for the H3 1-50 peptide trimethylated on K9 and K27 (H3K9me3K27me3) acquired from a middle down MS experiment. All other residues on this particular H3 peptide are unmodified. Relative intensity of 1.0 denotes the intensity of the most abundant ETD fragment ion in the spectrum, namely the c4 ion. Arrows indicate the c and z fragment ion peaks of a given charge state that were positively annotated. Hatched marks above and below the peptide sequence also correspond to the c and z fragment ions respectively found in the MS/MS.