Exceptional stories of microRNAs

MicroRNAs (miRNAs) are small, endogenous, non-coding RNAs that regulate the expression of target genes in eukaryotic cells and have been extensively studied in the past decade. However, recent evidence suggests that miRNAs have additional, important roles and functions other than post-transcriptional regulation through binding at the 3′ untranslated regions of their target genes. This review describes newly discovered information about the biogenesis and functions of miRNAs as well as presents different points of view about the miRNA system. Our increasing knowledge of the exceptional stories of miRNAs will offer new insights into these powerful gene regulators from virus and bacteria to animals.