Scientists at the Salk Institute have discovered a toggle switch for aging cells. By controlling the production of telomeres, it may be possible to keep healthy cells dividing and regenerating in old age. Most human cells can’t divide forever. Each time a cell divides, a cellular "timekeeper" at the ends of the chromosomes, known as a telomere, shortens. When the telomeres are shortened enough, cells are no longer able to divide, resulting in aging-related organ and tissue degeneration.