Neuroscientists have analyzed the diversity of inhibitory interneurons during the developmental period surrounding birth. They have discovered the emergence of three main sub-groups of interneurons by decoding the expression of cell-type specific genes as well as their exact, and often unexpected, location in the cortex. These results should help researchers in discovering how psychiatric-related genetic disturbances impact the emergence of neuronal sub-types and how to design novel cell-type specific interventions.