Strong guide strand bias, where the guide strand of the siRNA is selectively taken up into RISC over the passenger strand, is important both for maximizing siRNA silencing potency and for decreasing passenger strandrelated off-target effects. Although incorporating the right siRNA design parameters can help, siRNA design alone is not sufficient to ensure strong guide strand bias.


Luciferase reporter gene constructs with siRNA targets cloned in either the sense (guide strand target) or antisense (passenger strand target) orientation were co-transfected with the corresponding siRNA and a ß-galactosidase encoding control vector. Luciferase and ß-galactosidase assays were performed 72 hours later, and knockdown for each strand was calculated relative to negative control siRNA transfected cells. (A) The ratio of guide to passenger strand knockdown is shown for 46 siRNAs with and without the Silencer Select modifications. (B) The average ratio of guide to passenger strand knockdown is plotted versus luciferase knockdown for 6 Silencer Select and 36 competitor siRNAs.

The Chemical Modifications in Silencer Select siRNAs

  • Consistently enhance guide strand bias, which has been shown to correlate strongly with knockdown efficiency
  • Prevent the passenger strand from inducing silencing, which serves to reduce off-target effects
  • Result in no loss—and in many cases an improvement—in siRNA silencing potency