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An RNA whose folded structure binds a target with molecular specificity. Binding affinity can be expressed by a dissociation constant; it does not by itself fix binding and unbinding rates. Recognition can be coupled conceptually to an optical readout without altering the target's genetic material.

Reversible biomolecular association with a relatively high dissociation constant under specified conditions. The same equilibrium constant can result from different association and dissociation rates, so weak binding does not alone establish a short lifetime.