The adult hippocampus consists of the dentate gyrus (DG) and the CA1, CA2 and CA3 regions and is essential for learning and memory functions. During embryonic development, hippocampal neurons are derived from the ventricular zone and migrate to their final positions. Proper growth of the mammalian cerebral cortex, which is determined by expansion and survival of neural progenitors and mature neurons, is crucial for cognitive functions. Here, we show a role of the microRNA function in regulating the timing specific requirement of neural development.