Sialyltransferases add sialic acid to the terminal portions of the sialylated glycolipids (gangliosides) or to the N- or O-linked sugar chains of glycoproteins.

The biosynthesis of disaccharides, oligosaccharides and polysaccharides involves the action of hundreds of different glycosyltransferases.

These enzymes catalyse sialyltransfer reactions during glycosylation, and are type II membrane proteins.

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.

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.