Virtually all of the known enzymes utilize one, two, or three substrates to produce one or two products. The hyaluronan (HA) synthase enzyme family is very unusual, and perhaps unique, in that these synthases have two different enzymatic activities within the same protein. Furthermore, the growing HA chain produced after each sugar addition becomes the substrate for the next sugar addition. HA synthases are also membrane-bound enzymes. In bacteria and eukaryotic cells, active HA synthase is localized to the plasma membrane. The two activated sugar precursors for HA biosynthesis are UDP-glucuronic acid (UDP-GlcUA) and UDP-N-acetylglucosamine (UDP-GlcNAc). In order to make a disaccharide unit and extend the growing HA chain, HA synthase possesses at least six distinct functions. These activities include two different sugar nucleotide binding sites, two different glycosyltransferase activities, one or more binding sites for the growing HA chain, and the ability to transfer this HA chain within the enzyme, to set up the next round of saccharide addition. The UDP-sugar substrates are produced and used by the HA synthase inside the cell, and the HA chain is continuously transferred across the membrane so that it is extruded to the cell exterior. In the case of bacteria, this HA forms a capsule. In the case of many eukaryotic cells, the result is either the formation of a pericellular HA coat surrounding the cell or the release and deposition of HA into the surrounding extracellular matrix (ECM).