Small Interfering RNA (siRNA) is a class of double-stranded RNAs of 20-25 nucleotides that play important roles in many biological processes (Hamilton and Baulcombe, 1999). siRNAs act by “neutralizing” the mRNA of the target protein, facilitating degradation of the mRNA and hence altering the biological effect of the protein (reviewed in Hannon and Rossi, 2004). siRNAs may also change the intracellular levels of regulatory RNAs. Use of siRNAs for manipulating the expression of genes of interest in biological research is commonly referred to as RNA interference or knockdown technique (Elbashir et al., 2001). Synthetic siRNAs are an emerging tool that are now widely used in these studies. A variety of algorithms are employed by different companies for the design of siRNA products, which differ in efficacy, specificity and cost among other criteria. An example protocol of siRNA knockdown is explained here using the siGENOME SMARTpool reagents from Dharmacon.