The dynamic glycosylation of serine or threonine residues on nuclear and cytosolic proteins by O-Linked beta -N-acetylglucosamine (O-GLcNAc) is abundant in all multicellular eukaryotes, On several proteins, O-GLcNAc and O-phosphate alternatively occupy the same or adjacent sites, Leading to the hypothesis that one function of this saccharide is to transiently block phosphorylation, The diversity of proteins modified by O-GlcNAc implies its importance in many basic cellular and disease processes. Here we systematically examine the current data implicating O-GlcNAc as a regulatory modification important to signal transduction cascades.