Studying gene function using RNAi can be difficult unless you can identify your novel hits from your off-target effects. On his blog, Keith Robison suggests pooling siRNAs so their on-target effects are additive and the off-target ones are reduced. To minimize siRNAs from affecting one another, he also suggests having the siRNAs compete for a limited target. He also points his readers to a paper in Nucleic Acid Research in which the researchers find that Ago2 protein levels affect siRNA competition, as well as that RISC levels vary between cell lines.