It may look like a gyrating, Skittle-eating cloud, but it actually represents the active site of sulfite oxidase, an enzyme essential for normal neurological development in children. Look deep inside the channel opening to see a ball-and-stick model of a complex containing the element molybdenum (light blue ball) that helps pass an oxygen atom (red ball at far end of channel) to trigger a catalytic reaction. The channel itself is lined with a chloride ion (green) and water molecules (red). Knowing more about the enzyme's active site will help scientists better understand sulfite oxidase deficiency, a fatal neurological disease. Courtesy of bioinorganic chemist John H. Enemark at the University of Arizona.