Introduction to microRNAs in biological systems.

MicroRNAs are 20-24-nucleotide-long noncoding RNAs that bind to the 3' UTR (untranslated region) of target mRNAs. Since their discovery, microRNAs have been gaining attention for their ability to contribute to gene expression regulation under various physiological conditions. Consequently, deregulated expression of microRNAs has been linked to different disease states. Here, a brief overview of the canonical and alternative microRNA biogenesis pathways and microRNA functions in biological systems is given based on recent developments. In addition, newly emerging regulatory mechanisms, such as alternative polyadenylation, in connection with microRNA-dependent gene expression regulation are discussed.