(B) H2AX is ADP-ribosylated at glutamate 141 (E141). shPARG HCT116 cells were treated with H2O2 (2 mM in PBS, 5 minutes) and ADP-ribosylated residues were tagged by a hydroxamic acid derivative with an addition of 15.0109 Da, an increment that can be readily distinguished by mass spectrometry. Fragmentation of the NH2OH-derivatized peptides yielded typical b- and y-ion series, allowing easy localization of ADP-ribosylation sites. Glutamate 141 (E141) is indicated by an asterisk. 