The rate constants for macromolecular association reactions depend on the macromolecules involved and the solution conditions. The largest rate constants are for encounter-limited reactions, where association takes place whenever the associating macromolecules collide. Macromolecules that associate through specific binding sites often have much smaller rate constants. Recently, it has been demonstrated that the rate of association for complementary nucleic acid strands can be made encounter limited by attaching weakly associating, relatively unstructured polymeric domains to the nucleic acids involved. These results have led to a model suggesting that similar domains provide a general means of increasing rates of association for specific macromolecular binding partners, both in vivo and in vitro.