SiRNA buffer is recommended for long term storage, you can resuspend your SiRNA in RNAse free molecular grade water (or 0.22µm filtered DEPC treated water) for immediate use.

SiRNA buffer composition is usually available from your provider website, e.g. dharmacon 5X siRNA Buffer Composition: 300 mM KCl, 30 mM HEPES-pH 7.5, 1.0 mM MgCl2

It might be a good idea to check with the manufacturer, if siRNA oligos were dried down from a buffered solution. If so, you just need to add DEPC-treated water to reconstitute it in that buffer.

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