microRNAs are ligands of Toll-like receptors

It has long been known that microRNAs (miRNAs) can regulate target gene expression at the post-transcriptional level. Recent studies, however, have revealed that miRNAs can also be transported from donor cells to recipient cells, in which these RNAs function in a novel manner as ligands of Toll-like receptors. Here, we review the latest findings on these unconventional miRNAs, with special emphasis on their biological significance.