Researchers have identified a large population of previously unrecognized young neurons that migrate in the human brain during the first few months of life, contributing to the expansion of the frontal lobe, a region important for social behavior and executive function. These neurons likely contribute to inhibitory circuits, balancing the activity of their excitatory counterparts, and they may be associated with the plasticity of the brain that's observed during postnatal human development. Mercedes Paredes and colleagues analyzed postmortem brain samples from children younger than three months of age, tracking neurons expressing a marker associated with migration, doublecortin (DCX). Their postmortem analysis, in combination with MRI imaging in live infants, revealed a group of neurons that originates near the ventricles before migrating, postnatally, in an arc-like path into the frontal lobes. The number of these migratory cells decreased over the first seven months of age, the authors found; by two years of age, however, the migratory cells were no longer evident. The migrating cells primarily expressed markers associated with the inhibitory neurotransmitter γ-aminobutyric acid (GABA), suggesting the underlying neurons likely contribute to the establishment and maturation of local inhibitory circuits. In a related Perspective, Melissa McKenzie and Gord Fishel explain how the discovery of this neuron population helps account for "the massive enlargement of our frontal lobes, the hallmark of the brains of modern humans."