Hyaluronan (hyaluronic acid, HA) is composed of repeating disaccharide units of glucuronic acid and N-acetylglucosamine [-4GlcAb1-3GlcNAcb1-]. It is synthesized in the cell membrane by adding monosaccharides to the reducing end of the chain using the precursors UDP-glucuronic acid and UDP-N-acetylglucosamine in the presence of Mg2+. The integral membrane dual-action glycosyltransferase proteins hyaluronan synthases, of which vertebrates have three types (HAS1-3), catalyze these monosaccharide additions. Unlike other GAGs, HA is synthesized as a free glycan, not attached to a protein (Laurent 1987, Weigel & DeAngelis, 2007). As HA is synthesised it is extruded from the cell by an ABC-type transporter into the extracellular medium.