For generations, the adult nervous system's refusal to repair itself after injury has stood as one of medicine's most stubborn mysteries — not a question of whether healing was possible, but of what was preventing it. Researchers at the Icahn School of Medicine at Mount Sinai have now named the gatekeeper: a protein called AHR that compels injured neurons to choose survival over reconstruction, locking the body into a posture of endurance rather than renewal. When scientists removed or blocked AHR in mice, damaged nerve fibers began to regrow, and lost movement and sensation began to return. T