Noncoding RNAs: Emerging regulators of behavioral complexity.

The mammalian genome encodes thousands of non-coding RNAs (ncRNAs), ranging in size from about 20 nucleotides (microRNAs or miRNAs) to kilobases (long non-coding RNAs or lncRNAs). ncRNAs contribute to a layer of gene regulation that could explain the evolution of massive phenotypic complexity even as the number of protein-coding genes remains unaltered. We propose that low conservation, poor expression, and highly restricted spatiotemporal expression patterns-conventionally considered ncRNAs may affect behavior through direct, rapid, and often sustained regulation of gene expression at the transcriptional, post-transcriptional, or translational levels.