foetal environment can re-pattern development. We previously identified four microRNAs in foetal neural stem (NSC) and progenitor (NPC) cells, miR9, 21, 153 and 335, which were specifically suppressed by the neuroteratogen, ethanol [Sathyan et al., 2007, J. Neurosci. 27(32): 8546-8557]. Moreover, the sensitivity of these microRNAs collectively explained many of ethanol's effects on NSC renewal and maturation. We found that mir335 particularly, plays an important role in NSC/NPC survival and proliferation. MiR335 is coded within the MEST gene locus, which is implicated in the aetiology of the Russell-Silver syndrome, as well as in the spatial patterning of the emerging cerebral cortex. In situ hybridization and immuno-fluorescence analyses indicate that both mir335 and MEST localize to the apical cells of the fetal ventricular zone, further suggesting that the MEST/miR335 locus controls the proliferation and maturation of NSCs. To further study miR335's impact on NSC/ NPC maturation, we cultured foetal mouse neuroepithelial cells as non-adherent neurospheres. We used microarray approach to identify key genes targeted by miR-335. Our results indicate that 214 genes encompassing MAP Kinase, steroid biosynthesis and actin cytoskeleton reorganization pathways were significantly up-regulated following miR335 knockdown. Preliminary data indicates that FGF, which controls NSC renewal, also promotes the expression of miR335. These data collectively suggest that miR335 is an integral part of a complex signalling web that controls the growth and maturation of foetal NSCs. Its teratogen-sensitivity further supports the conclusion that microRNAs represent windows of vulnerability that render the developing foetus susceptible to environmental disrupters.