Short double-stranded RNA with sizes 19-29 bp can efficiently mediate gene silencing in mammalian cells by guiding the degradation of target mRNA sequences. These double-stranded RNA molecules—called short interfering RNA or siRNA—are created by expressing an RNA sequence that contains a fold-back stem-loop-stem structure—called a “hairpin.”  When small hairpin-type RNA sequences—also known as shRNAs—are transcribed in vivo, they are efficiently processed into functional double-stranded siRNA by cellular enzymes.  Alternatively, functional double-stranded siRNA constructs can be directly transfected into the cells, or synthesized from stably integrated vector constructs.  SBI's proprietary lentiviral siRNA vectors utilize opposing RNA polymerase III promoters, such as U6 and H1. 

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Different siRNA constructs targeting a specific mRNA vary widely in their efficiency.  Currently, empirical testing is the most accurate way to determine the efficiency of these effector molecules.  However, there are a number of selection criteria that are useful in designing siRNA sequences that are likely to inhibit expression with some efficacy.  Also, although synthetic versions of siRNA effectors have proved useful to suppress gene expression transiently, stable siRNA constructs that endogenously express the siRNA are the most effective in the study of gene function.  Using our FIV lentiviral system, we have shown that many sequences will provide significant expression inhibition of a specific target gene (click on the thumbnail to see the full image), proving that this approach is a fast and easy way to study gene function.