Ambion® Silencer® Select siRNAs are the result of recent improvements in siRNA design combined with novel chemical modifications yielding unrivaled knockdown efficiency and specificity. Our best performing siRNA for cell culture work. Silencer® siRNAs are unmodified 21 nucleotide siRNAs that have been designed with an older, yet rigorously tested design algorithm. A lower cost alternative to Silencer® Select siRNAs, but without some of the performance enhancements.