Sialyltransferases are enzymes that transfer sialic acid to nascent oligosaccharides. Each sialyltransferase is specific for a particular sugar substrate. Sialyltransferases add sialic acid to the terminal portions of the sialylated glycolipids (gangliosides) or to the N- or O-linked sugar chains of glycoproteins.

There are about twenty different sialyltransferases which can be distinguished on the basis of the acceptor structure on which they act and on the type of sugar linkage they form. For example, a group of sialyltransferases adds sialic acid with an alpha2,3 linkage to galactose, while other sialyltransferases add sialic acid with an alpha2,6 linkage to galactose or N-acetylgalactosamine. A peculiar type of sialyltransferases add sialic acid to other sialic acid units with an alpha 2,8 linkage, forming structures referred to as polysialic acid. As occurs for other glycosyltransferases, the expression of sialyltransferases undergoes profound modifications during cell differentiation and neoplastic transformation; in some cases such changes induce phenotypic alterations.^[1]

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