microRNA sponging by a new hearty circRNA

The discovery of circular RNAs (circRNAs) as a widespread and abundant class of endogenous RNA in Eukarya (1-3) has sparked a previously unparalleled interest in this class of RNA molecules. CircRNAs, characterized by a covalently closed structure, are generated through back-splicing of one or more exons from protein-coding and non-coding genes and have been found in great number in diverse species such as human (1-3) and Drosophila (4). Almost in parallel with the realization of circRNAs as pervasive endogenous molecules, came the first functional characterization of circRNAs; two circRNAs, termed ciRS-7/CDR1as and Sry circRNA were found to bind to miRNAs and prevent these miRNAs from acting on their cognate targets (2,5).