Glycosylation, consisting in incorporation of single N-acetylglucosamine residues attached by O-linkage to serine or threonine residues, is a common modification of nuclear proteins. Numerous chromatin and nuclear pore complex proteins as well as RNA polymerase II and some transcription factors are glycosylated in this unusual way. O-glycosylation of nuclear proteins has been postulated to play a role in nucleus-cytoplasmic transport, transcriptional regulation and regulation of protein phosphorylation level.