This family of glycosyltransferases includes O-linked beta-N-acetylglucosamine (O-GlcNAc) transferase, an enzyme which catalyzes the addition of O-GlcNAc to serine and threonine residues. In addition to its function as an O-GlcNAc transferase, human OGT also appears to proteolytically cleave the epigenetic cell-cycle regulator HCF-1.