MicroRNAs (miRNAs) and small interfering RNAs (siRNAs) act with the Argonaute family of proteins to regulate target messenger RNAs (mRNAs) posttranscriptionally. SiRNAs typically induce endonucleolytic cleavage of mRNA with near-perfect complementarity. For targets with less complementarity, both translational repression and mRNA destabilization mechanisms have been implicated in miRNA-mediated gene repression, although the timing, coupling, and relative importance of these events have not been determined. Here, we review gene-specific and global approaches that probe miRNA function and mechanism, looking for a unifying model. More systematic analyses of the molecular specificities of the core components coupled with analysis of the relative timing of the different events will ultimately shed light on the mechanism of miRNA-mediated repression.
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