The functions of long noncoding RNAs in development and stem cells.

Eukaryotic genomes are pervasively transcribed, with tens of thousands of RNAs emanating from uni- and bi-directional promoters and from active enhancers. In vertebrates, thousands of loci in each species produce a class of transcripts called long noncoding RNAs (lncRNAs) that are typically expressed at low levels and do not appear to give rise to functional proteins. Substantial numbers of lncRNAs are expressed at specific stages of embryonic development, in many cases from regions flanking key developmental regulators. Here, we review the known biological functions of such lncRNAs and the emerging paradigms of their modes of action. We also provide an overview of the growing arsenal of methods for lncRNA identification, perturbation and functional characterization.