**2. Guidelines for selecting potent siRNAs**

As discussed earlier, potency of siRNAs varies greatly with selection of target region. The parameters responsible for effectiveness of siRNAs being target accessibility, uniqueness of target region, absence of SNP sites etc. Other parameters worth considering while selecting the target site is the consideration of alternatively spliced isoforms of the target gene [5].

A parameter which greatly influences siRNA potency is RISC loading of the intended antisense strand [6]. Only one strand of the siRNA duplex enters into RISC and guides the complex to the target mRNA for its silencing. So, the choice of strand that enters into RISC complex plays the vital role in the whole silencing process. Generally RISC complex chooses the siRNA strand which has weaker 5' end binding energy. So this thermodynamic property should be ensured while selecting siRNAs. Also there are a number of sequence compositions that are said to be important for recognition of the intended strand by RISC and some parameters for enhancing the stability of siRNA duplex are also worth considering while selecting siRNAs. These rules can be divided into three categories 1) Selection of target region, 2) Structural and Thermodynamic Consideration and 3) Sequence Characteristics.
