Neurons and glial cells are constantly produced in small numbers in the adult CNS. In response to injury or tumor growth these numbers can increase dramatically. We discovered that TGF-β1, which is known to limit cell proliferation, is a potent inhibitor of both gliogenesis and neurogenesis in the aging mouse brain. TGF-β1 may have similar effects after injury and limit the potentially beneficial actions of newly generated cells. Current studies in the lab focus on other innate immune molecules as regulators of neurogenesis.