Aptamers that bind specific targets are identified through a process known as Systematic Evolution of Ligands by Exponential enrichment (SELEX)
in which binding molecules are selected from a large and diverse library of nucleic acids (either DNAs or RNAs).
In this process, the nucleic acid library is incubated with the target molecule.
Non-binding nucleic acids are then washed away, leaving behind only the molecules that have a capacity to bind to the target molecule.
The nucleic acids that are not washed away are then used to create a new library of nucleic acids that is enriched for the subset that binds the desired target.
Repeating this selection-cycle on each subsequent library with increasing stringency of binding (e.g. lower concentration of target),
nucleic acids that bind to the target with both high specificity and high affinity are enriched.