Rapamycin is an immunosuppressant drug that prevents rejection of transplanted organs, but researchers now find that it not only suppresses immune reactions but also extends the life span of mice (Nature, DOI: 10.1038/nature08221). Rapamycin was already known to increase the life span of invertebrates such as fruit flies but not that of mammals. David E. Harrison of the Jackson Laboratory, in Bar Harbor, Maine, and coworkers report that rapamycin helps male mice live on average 9% longer and female mice 14% longer when it is administered beginning at about 20 months of age—relatively late in life for mice. Rapamycin works by inhibiting the receptor mTOR, and the researchers note that their results appear to be the first "to demonstrate a role for mTOR signaling in the regulation of mammalian life span, as well as pharmacological extension of life span in both genders." Rapamycin analogs might prove to be useful life-extension agents in people, but rapamycin itself shouldn't be taken to improve life span because it has harmful side effects.