This enzyme, found on the surface of avian influenza viruses, degrades sialic acid residues in the thin layer of mucus that lines the airways to our lungs, allowing the virus to cross this protective barrier and attack the vulnerable epithelial cells that lie just underneath. In its biologically active form, neuraminidase is a large, mushroom-shaped structure combining an enzymatically-active cap supported by a long stalk projecting from the viral surface. This image shows four neuraminidase cap subunits from an avian influenza virus, along with their associated carbohydrate moieties (small dun-colored molecules). The sialic acid binding sites are visible as single cavities in each of the subunits. The image is a digital computer model based on real molecules. Magnification: 20 million times at 7"x7" print size.