Proteomic strategy for histone PTM quantification. Each of the 24 lines was similarly processed to yield the core and linker histones. Histones were derivatized with propionic anhydride to prevent trypsin digestion at the unmodified and monomethylated lysines and, consequently, peptides of uniform length are produced that span the same modified residue (in this example, lysines 9 and 14 of histone H3). Relative quantification is achieved by peak integration of the extracted ion chromatograms of each charge state (the +2 charge state is shown) for each peptide (each colored row, with m/z shown). Note that H3K9acK14un and H3K9unK14ac co-elute in this experiment, thereby requiring tandem MS sequencing to resolve both peptides for separate quantification.