The identification of progenitor cell populations capable of producing neurons and defining the intrinsic and extrinsic factors that regulate neuronal progenitor cell proliferation have been subjects of increasing interest over the last decade. Two major reasons for this emphasis are the gradual acceptance that at least some regions of the brain generate neurons throughout life and the growing appreciation that neuronal progenitor cells could be used to therapeutically treat disorders and injuries of the central nervous system (CNS) (1). Despite early studies by Altman and Das (2) demonstrating ongoing neurogenesis in the adult rodent hippocampus and olfactory bulb, it was widely believed until recently that in primates the generation of neurons ceases in the late embryonic or early postnatal period (e.g., ref. 3). Although it is certainly true that a large majority of neurons are generated prenatally or in the early postnatal period, strong evidence that new olfactory bulb neurons are produced throughout life in primates, as well as lower mammals, is discussed below, and there is also evidence of ongoing neurogenesis in the primate hippocampus (4,5). In addition, olfactory receptor neurons, the first-order neurons in the olfactory system, have been found to regenerate throughout life in all vertebrates examined (6,7). Thus, the capacity, and indeed, the fact of neurogenesis continuing throughout life in the CNS of higher, as well as lower, mammals is now well established.