The shaping of neuronal circuits is essential during postnatal brain development. A window of neuronal remodeling by sensory experience typically occurs during a unique time in early life. The many types of behavior and perception, like human language, birdsong, hearing and vision are refined by experience during these distinct 'critical periods'. The onset of critical periods for vision is delayed in animals that remain in complete darkness from birth. It is then predicted that a 'messenger' within the visual pathway signals the amount of sensory experience that has occurred. Our recent results indicate that Otx2 homeoprotein, an essential morphogen for embryonic head formation, is reused later in life as this 'messenger' for critical period plasticity. The homeoprotein is stimulated by visual experience to propagate into the visual cortex, where it is internalized by GABAergic interneurons, especially Parvalbumin-positive cells (PV-cells). Otx2 promotes the maturation of PV-cells, consequently activating critical period onset in the visual cortex. Here, we discuss recent data that are beginning to illuminate the physiological function of non-cell autonomous homeoproteins, as well as the restriction of their transfer to PV-cells in vivo.