These six-month-old axolotls—a kind of salamander—glow green under blue light. That's because they were genetically modified to make harmless green fluorescent protein, or GFP. Like X-ray vision, GFP lets you see inside the axolotls as they hang out in their aquarium. GFP not only can reveal internal structures in living organisms, but it also can light up specific cells and even proteins within a cell. That allows scientists to identify and track things like cancer cells. GFP's abilities have made it one of the most useful tools in biomedical research and earned its discoverers a Nobel Prize. Courtesy of Jill Grossman, Jamison Hermann and Marc Zimmer, Connecticut College (music: "spinnin" by grapes).

Featured in the November 18, 2009, issue of Biomedical Beat.