Figure 1: Structure of an siRNA: 21 bp RNA duplex with 2 nucleotides 3′′ overhanging on each strand; the two strands are called antisense or active or guide strand and sense or inactive or star strand, respectively; the first 2–8 bases of the antisense strand are called seed region, and at bases 8–10 of the antisense siRNA strand is the cleavage site.