p19–siRNA complex structure. a, A stereo view perpendicular to the two-fold axis. Individual monomers of the p19 dimer are coloured blue and magenta, while the two siRNA strands are coloured orange and pink. Two tryptophans from each monomer of the p19 dimer, which bracket the terminal base pairs at either end of the siRNA duplex, are shown in stick representation. b, c, Alternative mono views of the complex rotated by 90° along a different axis. d, RNA-contacting region (green) along the dimeric protein surface. e, Electrostatic surface of dimeric protein, with blue and red colours corresponding to positively and negatively charged patches.