AbstractDicer, an RNase III endonuclease, is the enzyme which cleaves microRNA (miRNA) and small interfering RNA (siRNA) precursors into 21–25 nucleotide species. This cleavage is an essential step in the biogenesis of these small noncoding RNA molecules. In their mature forms, siRNA and miRNAs function to regulate gene expression through different mechanisms (Bartel, 2004). To investigate the role of Dicer and microRNAs in neuronal polarity development, we used mice in which the RNase III domain of Dicer was conditionally floxed. To knockout Dicer gene, hippocampal neurons were electroporated with Cre together with pmaxGFP®plasmid by Amaxa®Mouse Neuron Nucleofector®Kit. Neuronal polarity was analyzed at 3 days in vitro (DIV). Neurons expressing pmaxGFP®showed normal polarity. In contrast, the majority of neurons transfected with Cre developed multiple axons. We found multiple axons were significantly increasing. Here we explore Dicer function in neuronal polarity by inactivating it in the hippocampal neuron using the Cre/loxP approach. Neurons which lack Dicer have multiple axons, demonstrating that Dicer is essential for neuron polarity, providing evidence that Dicer function is required to neuronal development.