Circular RNAs (circRNAs) have been detected in animal cells, but their function has been unclear. Two papers, from the laboratories of Nikolaus Rajewsky and Jørgen Kjems, have now defined a function for one circRNA that binds the microRNA miR-7. They find that this circRNA is replete with microRNA binding sites and can act as a 'sponge' capable of binding scores of miRNAs per circRNA molecule. These studies suggest a role for circRNAs in post-transcriptional regulation.
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