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Why is there an AA at the 5'-end of published siRNA sequences?

The AA historically represented the leader sequence of the siRNA target. These nucleotides are not synthesized as part of the siRNA but are part of the nomenclature used by researchers in the field. The leader sequence identifies the sequence corresponding to the two-base overhangs on the 3'-ends of the siRNA. Often siRNA targets are published as AA-(N19) to indicate that the overhang composition is either dTdT or UU. When placing an order, the N19 target sequence should be indicated followed by selection of the desired two-base overhang composition, resulting in a 21-mer duplex.