Sometimes mice get all the breaks. Scientists working with an immunosuppressing compound called rapamycin have found that it can extend the lives of mice an average of 9% for males and 13% for females. The study, conducted by the National Institute on Aging's Interventions Testing Program, used mice that were about 600 days old well into mouse middle age. The treated male mice got a maximum of 101 extra days; the females got 151. How rapamycin works is not clear, but the investigators believe it may be related to the mechanics of caloric restriction, which has also been shown to have life-extending powers. The drug may trick cells into thinking they've been depleted of nutrients, prodding them to use proteins more efficiently. Is there human-life extension in rapamycin's future? Probably not. Earlier human trials showed that it can lead to opportunistic infections, high triglycerides and heart disease. Rapamycin may work better as therapy for age-related ills like Alzheimer's disease.