Sialyltransferases catalyze the transfer of sialic acid from cytidine 5-prime monophospho-N-acetylneuraminic acid (CMP-NeuAc) to terminal positions of glycoprotein and glycolipid carbohydrate groups. Terminal NeuAc residues are key determinants of carbohydrate structures, such as the sialyl-Lewis X determinants, and are widely distributed in many cell types. However, cancer cells often express more heavily sialylated glycans on their cell surface and this feature sometimes correlates with invasiveness. In contrast, expression of ST3gal6, a member of the sialyltransferase family that sialylates type II lactosamine structures on glycoproteins and glycolipids, was found to be significantly decreased by hypermethylation of the gene in gastrointestinal cancer. At least three isoforms of ST3gal6 are known to exist.