Before monosaccharide units are incorporated into glycoproteins, polysaccharides, or lipids in living organisms, they are typically first "activated" by being joined via a glycosidic bond to the phosphate group of a nucleotide such as uridine diphosphate (UDP), guanosine diphosphate (GDP), thymidine diphosphate (TDP), or cytidine monophosphate (CMP). These activated biochemical intermediates are known as sugar nucleotides or sugar donors. Many biosynthetic pathways use mono- or oligosaccharides activated by a diphosphate linkage to lipids, such as dolichol. These activated donors are then substrates for enzymes known as glycosyltransferases, which transfer the sugar unit from the activated donor to an accepting nucleophile (the acceptor substrate).