DALLAS, United States -- Dallas scientists have created mice that stretch the limits of knowledge. Or just stretch. The mice lack a key protein that makes certain cells and tissues of the body elastic. Without this one cell part, the mice ended up with droopy skin, ailing lungs, and loose blood vessels. The researchers will study the mice to learn more about what happens as an aging human body loses elasticity. We know that skin gets saggy and drawn. And in the lungs, airways that lose springiness can't inhale and exhale correctly, causing emphysema. The mouse studies may help figure out why. "The elastic properties of tissues are of fundamental importance in health and disease," said Eric Olson of the University of Texas Southwestern Medical Center at Dallas. He described the creation of the mice last week in the journal Nature. The scientists made the mice through genetic engineering, by shutting down a gene called fibulin-5. They didn't know they would end up with baggy mice. They only knew that the gene contained the instructions to make a protein that was particularly active in the skin, lungs, and blood vessels. "Just knowing where someone lives and what they look like doesn't tell you what they do for a living," said Kenneth Chien of the University of California, San Diego. Dr. Chien and his colleagues, working separately, also created the same kind of mouse. The California paper also appeared last week in Nature. The mice at first seemed normal. But after about four weeks, their jowls sagged and skin drooped. The scientists investigated the mice and found that without fibulin-5, springy proteins in cells can't attach themselves correctly. The next step is to see what role a malfunctioning fibulin-5 might play in human disease. In addition to Dr. Olson, Hiromi Yanagisawa, Elaine Davis, Takashi Ouchi, Masashi Yanagisawa, and James Richardson of UT Southwestern worked on the research, as did Barry Starcher of the University of Texas at Tyler.