The O-antigen is a repetitive glycan polymer with a repeat unit of two to six sugar residues, and comprises the outermost and immunogenic domain of the lipopolysaccharide (map00540). Most of the sugar residues are derived from nucleotide sugars and the genes for their biosynthesis seem to be well conserved among Gram-negative bacterial species. They include UDP-sugars synthesized from Glc-1P via UDP-Glc or from GlcNAc-1P via UDP-GlcNAc, GDP-sugars synthesized from Fru-6P via GDP-Man, and dTDP-sugars and CDP-sugars synthesized from Glc-1P via dTDP-Glc and CDP-Glc, respectively. CMP-sugars are also synthesized from UDP-GlcNAc. The O-antigen nucleotide sugar biosynthesis genes often form gene clusters with glycosyltransferase genes and other processing genes such as for translocation and polymerization.