Changes in polarity and morphology have been more extensively studied in vitro. In hippocampal neurons in culture, a particularly early event in neuronal polarization is the segregation of activatable, membrane inserted, IGF-1R to one neurite in neurons that do not yet exhibit a discernible axon (stage 2 of differentiation10). Subsequently, phosphatidylinositol-3 kinase (PI3K) is activated and its product, PIP3, accumulates in the distal region of the neurite, together with IGF-1R. These events are critical for the outgrowth of the future axons and the establishment of neuronal polarity10–12. Similarly, IGF-1R activation was reported to be necessary for the regulation of axonal outgrowth of motor neurons13. However, a possible role of the IGF-1R in neuronal migration and the establishment of polarity in the cortex has not been addressed.