Authors Tsai and Barnea report in the most recent issue of Science that a critical period exists during which an olfactory “sensory map” becomes established. Previous findings show that the olfactory system exhibits enhanced plasticity that is maintained throughout life due to continuous neurogenesis of sensory neurons in the nose and olfactory bulb. However, the authors found evidence in mice that there is less plasticity than originally thought and that a sensory map is created during a developmental critical period. Specifically, the authors were able to test the neural pathways of transgenic mice, finding that perinatal expression of the transgenic odorant receptor (i.e. before the sensory map is established) led to activation of new axons. In contrast, expression of the transgenic receptor after the sensory map was established did not activate new axons.