Researchers agree that telomeres play a role in aging, but many caution that telomere shortening cannot entirely explain the process. For example, mice have longer telomeres than humans but a far shorter life span. "The data is not there one way or another," says Sheila Stewart, a cancer biologist and telomere researcher at the Whitehead Institute of the Massachusetts Institute of Technology. Aging results from the interaction of many factors, she speculates, of which telomeres are only one. Though researchers don't believe in a fountain of youth, the search for clues about aging (and cancer) continues.