ETD-MS 2 spectrum of the doubly O- GalNAcylated peptide from the GalNAc-T7 reaction. The mass of c- and z - fragment ions (e.g. the delta mass between c11 and c13) indicated the second GalNAc residue is located at S686. The GalNAc residues are denoted as yellow squares according to Consortium for Functional Glycomics (CFG) standard. Source data are provided as a Source Data file.
