Why do Brad Pitt and Angelina Jolie look the way they do and we, um, don't? One reason could be a little DNA sequence clustered near a human gene called TERC. The TERC gene is already known to produce an enzyme called telomerase, which helps regulate the length of telomeres — caps at the end of chromosomes similar to the plastic tips at the ends of shoelaces. Every time a cell divides, telomeres shorten, leading to a chromosomal fraying associated with aging. In a British study published in the journal Genetics, scientists found that people with one copy of the gene had slightly shorter telomeres similar to those of people three or four years older who didn't carry the gene; in other words, they were aging three or four years faster. In another study in Nature, researchers at Harvard Medical School were able to switch on a telomerase gene in prematurely aged mice, and reverse the aging process; the mice's organs regnerated, their shrunken brains increased in size, and their fertility was restored. Manipulating the gene for telomerase could, in theory, slow aging or at least the development of age-related diseases in humans. But there's reason for caution: Rapidly dividing, semi-immortal cells are also known as cancer cells, meaning that the search for eternal youth could yield an entirely different — less pleasant — outcome.