From Science News: "the enzyme telomerase can extend the lifespan of mice by about 24 percent. ... Telomerase lengthens telomeres - the 'caps' on the end of chromosomes that protect DNA from damage. Like burning fuses, telomeres normally get shorter each time that most body cells divide. ... While the enzyme enables cells to keep dividing, it also takes cells one step closer to growing and proliferating out of control - that is, becoming cancerous. Lab animals with extra genes for telomerase often die young from tumors. ... [researchers] engineered mice to have not only an extra copy of the gene for telomerase, but also extra anti-tumor genes to combat the enzyme's cancer-causing potential. In the altered mice, signs of aging such as poor coordination or degraded tissue health were delayed compared to mice that had only the extra copies of anti-tumor genes." Most interesting; you might also want to look at recent research that suggests telomerase operates by protecting mitochondria, and less damaged mitochondria means better preservation of telomeres - but, more importantly for life span, less oxidative stress.