O-GlcNAc (O-linked N-acetylglucosamine) is one of the most abundant posttranslational modifications on nuclear and cytoplasmic proteins. Many cellular proteins, including nuclear pore, oncogene, cytoskeletal, heat shock, viral and transcription regulatory proteins contain single O-GlcNAc residues attached to serine or threonine residues. This modification occurs via the O-GlcNAc transferase (OGT). There appears to be a competitive modification at these residues. Either the residues are glycosylated or they are phosphorylated. Because of the significance of phosphorylation in cancer research, such competition can be of major interest. O-GlcNAc bearing proteins tend to be found in multimeric complexes. This has led to the suggestion that O-GlcNAc glycosylation may also obscure phosphorylation sites and acts as a signaling mechanism or mediator of signaling. Recent research has revealed that the O-GlcNAcylation of the EGF repeats of Notch are modified via a novel O-GlcNAc transferase, EOGT1.