Guanosine diphosphate mannose (GDP-mannose) is a nucleotide sugar substrate for glycosyltransferase (mannosylation) reactions mediated by mannosyltransferases. GDP-mannose may be used to study its transport, mannose metabolism, and presence as a component of the sugar nucleotide pools of various cell types. GDP-mannose is used as a precursor ofL-ascorbic acid in plants.