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Postmitotic neurons in the neocortex migrate to appropriate positions and form layered structures of nascent cortex during brain development. The migration of these neurons requires precise control and coordination of a large number of molecules such as axon guidance cues. The Eph-ephrin signaling pathway plays important roles in the development of the nervous system in a wide variety of ways, including cell segregation, axon pathfinding, and neuron migration. However, the role of ephrin-B2/EphA4 signaling in cortical neuron migration remains elusive. Here we demonstrated that ephrin-B2 and its receptor EphA4 were expressed in complementary and overlapping patterns in the developing neocortex. Deletion of the EphA4 gene in the embryonic cerebral cortex resulted in faster migration of cortical neurons, whereas knockdown or overexpression of ephrin-B2 did not alter the normal process of migration. These results suggest that ephrin-B2 forward signaling through EphA4 is required for the precise control of cortical neuron migration. [ABSTRACT FROM AUTHOR]