Systematic Evolution of Ligands by Exponential Enrichment (SELEX) is used to identify nucleic acids with high affinities and specificities to a distinct target ligand. Frequent targets include small molecules, proteins/peptides, and carbohydrates. Aptamers frequently possess 2'-fluoro, 2'-O-methyl, phosphorothioates or 5-[(3-Indolyl)propionamide-N-allyl]-2'-deoxyuridine.