Protein O-GlcNAc transferases catalyze the transfer of N-acetylglucosamine (GlcNAc) from UDP-GlcNAc to the oxygen atom of serine/threonine hydroxyl groups of proteins. The OGT protein has this activity for intracellular proteins, while the EOGT protein specifically catalyzes O-GlcNAcetylation of EGF repeats on certain secreted/membrane proteins.