Researcher Ronald DePinho, director of the Belfer Institute of the Dana-Farber Cancer Institute, and colleagues at Harvard Medical School engineered mice that did not produce the enzyme telomerase, a chemical which protects the ends of DNA strands from being lost during replication. In an earlier study, these mice died after only six months, compared to the three-year life span of a normal mouse. But in DePinho's recent study, a chemical fountain of youth was given to the mice when they received special form of estrogen. The mice were engineered to produce telomerase only when this special estrogen was present.