The Click-iT™ O-GlcNAc Enzymatic Labeling System provides a highly sensitive and efficient method for the in-vitro modification of O-GlcNAc modified proteins. Proteins are enzymatically labeled utilizing the permissive mutant β-1,4-galactosyltransferase (Gal-T1 (Y289L)) which transfers azido-modified galactose (GalNAz) from UDP-GalNAz to O-GlcNAc residues. Then, via the chemoselective ligation or click reaction between an azide and an alkyne, the azido-labeled glycoproteins can then be detected with a Click-iT™ Glycoprotein Detection kit for gels (TAMRA or Dapoxyl™ alkyne) or Western blots (biotin alkyne). Labeling and detection can be completed in less than 24 hours and the Click-iT™ glycoprotein products are compatible with LC-MS/MS and Multiplexed Proteomics™ technologies for in-depth analyses of this important post-translational modification.