Abstract
MicroRNAs (miRNAs) are major regulators of gene expression in plants and animals. These small RNAs function within RNA-protein complexes to silence messenger RNAs targeted for repression. We have determined structures of the human proteins Argonaute-2 and TNRC6, two core components of miRNA-silencing complexes. Our results provide insights into how miRNAs identify target RNAs, and specifically sequester them for degradation.