<p>Aptamers are single-stranded nucleic acid ligands that recognize and bind to targets with high affinity and selectivity.  They are isolated from large combinatorial libraries containing approximately 10^15 different sequences. Selection from these libraries is done in vitro using the Systematic Evolution of Ligands by Exponential enrichment (SELEX) process. Made from DNA, RNA or is some cases modified nucleotides, aptamers consist of linear stretches of 15 or more nucleotides in length. However, it is their ability to fold into specific 3D conformations that allow tight binding to their target. These ligands display binding affinities that rival antibodies. For instance, aptamers have been found that bind to proteins with dissociation constants in the picomolar range.  Other targets for which aptamers have been developed include small organic dyes, toxins, viruses and whole cells.  Aptamer-target interactions are also highly selective. Aptamers can distinguish their targets on the basis of subtle structural differences such as chirality or the presence or absence of a single methyl group. For these reasons, aptamers have become an attractive molecular recognition agent in the development of diagnostics and therapeutics.</p>