Schratt et al. found that the brain-specific miRNA, miR-134, is localized to the synaptodendritic compartment and negatively regulates the size of dendritic spines (Schratt et al., 2006). This action of miR-134 occurs through its inhibitory action on lim1 kinase expression. Another brain-enriched miRNA, miR-124, has been shown to play a critical role in the transition of progenitor neuronal cells to adult neurons by inhibiting networks of non-neuronal genes, thereby facilitating the expression of the neuronal identity (Conaco et al., 2006; Makeyev et al., 2007). Aizawa and colleagues examined the effects of double deletion of miR-9-2 and miR-9-3 on brain development in mice (Shibata et al., 2008, 2011). They found that these miR-9 family members regulate the proliferation and differentiation of neural progenitor cells in telencephalon through inhibitory actions on regulator proteins important for neurogenesis, including the homeobox protein Meis2 and the transcription factor Forkhead box protein G1 (FOXG1) (Shibata et al., 2008, 2011). Other miRNAs shown to regulate neuronal lineage commitment include members of the let-7 family and miR-125b (Leucht et al., 2008; Rybak et al., 2008). More recently, hippocampus-expressed
