siRNA design principles and off-target effects.

Short interfering RNAs (siRNAs) are a major research tool that allows for knock-down of target genes via selective mRNA destruction in almost all eukaryotic organisms. siRNAs typically consist of a synthetic ∼21 nucleotide (nt) RNA-duplex where one strand is designed with perfect complementarity to the target mRNA. Although siRNAs were initially thought to be very target-specific because of their design, it turned out during the last years that all siRNAs have a more or less pronounced intrinsic off-target activity which can make the interpretation of data from siRNA experiments difficult. Here we describe essential rules for siRNA design that should be taken into account in order to obtain potent siRNAs with minimal off-target activity. In addition, we describe how to control for off-target activity in siRNA experiments.