[
Nature Neuroscience,
2004]
At first glance, the nervous systems of vertebrates and invertebrates seem bilaterally symmetrical, but on closer inspection left-right asymmetries become apparent. Humans, for example, show gross anatomical differences between right and left temporal lobes, and visual and language faculties are asymmetrically distributed between the two hemispheres. How these asymmetries arise during development remains something of a mystery (for review, see ref.1). In the nematode Caenorhabditis elegans, the AWC and ASE chemosensory neuron pairs are bilaterally symmetrical based on anatomical considerations, but nevertheless display asymmetrical gene expression patterns. A recent study in nature by Johnston and Hobert identifies a microRNA (miRNA) as a crucial mediator of this asymmetry in the ASE neurons.