Scientists had previously engineered C. elegans worms to alter signaling pathways for a protein known as insulin-like growth factor 1 (IGF-1) and found that they lived about five times longer than normal worms. The increased lifespan had its drawbacks, however: it left the worms very lethargic. Cynthia Kenyon of the University of California at San Francisco and her colleagues perturbed genes in C. elegans that affect the activity of insulin and removed gonad tissue, which affects endocrine hormone levels. Worms treated this way lived six times longer than normal worms and remained active for most of their lives, according to the report. "These life-span extensions, which are the longest mean life-span extensions ever produced in any organism, are particularly intriguing," the team writes, "because the insulin/IGF-1 pathway controls longevity in many species, including mammals."