A new class of nucleic acid compounds, referred to as nucleic acid ligands, have been shown to exist that have a specific binding affinity for three dimensional molecular targets. In a preferred embodiment the nucleic acid ligands are identified by the method of the invention referred to as the Systematic Evolution of Ligands by EXponential enrichment (SELEX), wherein a candidate mixture of nucleic acids are iteratively enriched in high affinity nucleic acids and amplified for further partitioning.