MS/MS spectra of INEILSNALKR with a Lys residue modified with SUMO1 or SUMO3 remnant chains. MS/MS spectra of INEILSNALKR with a Lys residue modified with SUMO1 or SUMO3 remnant chains. Product ion spectrum for [M+3H]3+ (m/z 581.651 for SUMO1, m/z 576.652 for SUMO3) acquired with ETD (A, B), CID (C, D) and HCD (E, F). SUMO-specific fragment ions (e.g. c1*, b2*, b2*-H2O, b3*, b3*-H2O) and neutral losses are emphasized in black while other fragment arising from the peptide backbone are shown in gray. Frederic Lamoliatte et al. Mol Cell Proteomics 2013;12:2536-2550 © 2013 by The American Society for Biochemistry and Molecular Biology, Inc.