Fucose (CAS: 2438-80-4) is a hexose deoxy sugar with the chemical formula C6H12O5. L-Fucose (6-deoxy-L-galactose) is a monosaccharide that is a common component of many N- and O-linked glycans and glycolipids produced by mammalian cells. It is the fundamental subunit of the fucoidan polysaccharide. As a free sugar, L-fucose is normally found at very low levels in mammals. It is unique in that it is the only levorotatory sugar synthesized and utilized by mammals. Fucose polymers are synthesized by fucosyltransferases. All fucosyltransferases utilize a nucleotide-activated form of fucose, GDP-fucose, as a fucose donor in the construction of fucosylated oligosaccharides. The ABO blood group antigens are among the most well known fucosylated glycans. The alpha-1->3 linked core fucose is a suspected carbohydrate antigen for IgE-mediated allergy. Two structural features distinguish fucose from other six-carbon sugars present in mammals: the lack of a hydroxyl group on the carbon at the 6-position (C-6) and the L-configuration. In fucose-containing glycan structures, fucosylated glycans, fucose can exist as a terminal modification or serve as an attachment point for adding other sugars. Fucose is metabolized by an enzyme called alpha-fucosidase. Fucose is secreted in urine when the liver is damaged. Free L-fucose in serum and urine can be used as a marker for cancer, cirrhosis, alcoholic liver disease and gastric ulcers. Elevated levels of serum fucose have been reported in breast cancer, ovarian cancer, lung cancer, liver cancer, diabetes, and cardiovascular disease. It has been shown that feeding rats a diet high in L-fucose induces neuropathy similar to that seen in diabetics.