We describe a simple innovation that allows DNA double helices to stably bind one another at specific sites, with regulatable affinity, under physiological conditions. This type of DNA synapsis requires neither an unraveling of the participating duplexes not heteroduplex formation, and is achieved by the intermolecular dimerization of short blocks of guanine-guanine mismatch base-pairs introduced within standard Watson-Crick duplexes. We propose that in vivo such "sticky" guanine domains, formed transiently in cruciforms, could initiate illegitimate recombination events. In practical terms, this type of synapsis, achievable in vitro by simply mixing the participating duplexes, could provide a novel and general technology for the self-assembly of arrays of important DNA sequences, and serve as a tool for investigating certain protein-DNA interactions in vivo.