O-linked N-acetylglucosamine (O-GlcNAc) was discovered during studies using bovine milk galactosyltransferase to ‘map’ the surface topography on cells of the murine immune system (Torres and Hart, 1984). Later, O-GlcNAc was shown to reside almost exclusively in the nucleus and cytoplasm (Kearse and Hart, 1991b), and to be present in eukaryotes ranging from trypanosomes, yeast, plants, to man, as well as in viruses (Hart et al. 1989; Hart et al. 1994a; Greis and Hart, 1994). We now know that O-GlcNAc is an exceedingly abundant post-translational modification of specific serine/threonine residues of many important nuclear and cytoplasmic proteins (Haltiwanger et al.1992b). Figure 1 lists the O-GlcNAc-bearing proteins identified to date. O-GlcNAc is attached as a monosaccharide and is generally not further modified. The O-GlcNAc turn-over rates are typically many-times that of the polypeptide backbone on which it is found (Chou et al.1992a; Hart and Roquemore, unpublished). The saccharide is attached at sites similar to those also used by the ‘growth-factor’, proline-directed family of kinases (Roach, 1991; Taylor and Adams, 1992). O-GlcNAc-bearing proteins have a diverse range of functions, but are characterized by several common features: 1) They all are also phosphorylated. 2) They typically form specific and regulated multimeric associations with other polypeptides. 3) In several cases, O-GlcNAcylation and phosphorylation appear to be reciprocal events.