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Sialic acids are a family of closely related nine-carbon carboxylated sugars found at the non-reducing terminal position of mammalian cell surface sugar chains of glycoproteins, glycolipids and oligosaccharides. They are glycosidically linked to either the 3-or 6-hydroxyl groups of galactose residues (Gal), or to the 6-hydroxyl group of A'-acetylglucosamine (GlcNAc) or N-acetylgalactosamine (GalNAc) residues, and can form polysialic chains via their 8-hydroxyl group and terminate with a sialic acid branched via the 8-or 9-hydroxyl group. Because of their terminal position and their charge, sialylated oligosaccharide sequences have long been predicted to be information-containing molecules and critical determinants in cell-cell recognition processes (Rademacher et ai, 1988) and, indeed, for a very long time, cell surface sialic acid residues have been known to be involved as receptors for influenza virus (Rogers et …