The protein mTOR is one of many flagged as important to longevity by studies of calorie restriction biochemistry. Here it is implicated in the decline in stem cell function with age: "Age-related declines in hematopoietic stem cell (HSC) function may contribute to anemia, poor response to vaccination, and tumorigenesis. Here, we show that mammalian target of rapamycin (mTOR) activity is increased in HSCs from old mice compared to those from young mice. mTOR activation through conditional deletion of Tsc1 in the HSCs of young mice mimicked the phenotype of HSCs from aged mice in various ways. These [included] a relative decrease in lymphopoiesis [and] impaired capacity to reconstitute the hematopoietic system. In old mice, rapamycin increased life span, restored the self-renewal and hematopoiesis of HSCs, and enabled effective vaccination against a lethal challenge with influenza virus. Together, our data implicate mTOR signaling in HSC aging and show the potential of mTOR inhibitors for restoring hematopoiesis in the elderly."