Cover picture: A model for RecA-promoted strand exchange via parallel triple-stranded helices. (Left) Starting triplex. The invading strand (blue) occupies the minor groove of the Watson-Crick paired strands. (Center) From top left to bottom right: evolution of the base pairing within a GCxG triplet during strand exchange. (Right) Resulting triplex. The leaving strand (red) occupies the major groove of the duplex. See the article by Bertucat et al. on p. 1562.