Introduction We have described a major new form of protein glycosylation (termed 0-GlcNAc) that is found in all eukaryotes from yeast to man. 0-GlcNAc is distinct from other types of protein-bound saccharides in several respects (for review see [ l ] ) : (1) 0-ClcNAc is a simple, unmodified monosaccharide moiety glycosidically linked to the side-chain hydroxyls of serine or threonine, often occurring at multiple sites on the same protein. (2) Virtually all 0-GlcNAc is found within the nucleoplasmic and cytoplasmic compartments of cells. (3) 04;lcNAc appears to be highly dynamic and responsive to cellular stimuli in a fashion analogous to phosphorylation. In this short review, we will first outline evidence localizing 0-GlcNAc to the nucleoplasmic and cytoplasmic cellular compartments and discuss proteins that bear 0-GlcNAc. Subsequently, we will illustrate what is known about the sequence localization of 0-GlcNAc attachment sites and the enzymes that specifically attach or remove the saccharide. Finally, we will describe our working hypotheses about the functions of this ubiquitous eukaryotic protein modification.