Interactions of proteins with nucleic acids play important roles in biological phenomenon. Almost every stage in the regulation of gene expression involves the interaction of proteins with specific nucleic acids sequences. The identification of the nucleic acid recognition sequence of a given DNA-binding protein is therefore often the first step to be undertaken in the study of its biological function. Over the last 10 yr, the SELEX procedure (systematic evolution of ligands by exponential enrichment) has been used to identify high-affinity nucleic acids ligands for a large number of different proteins. The method was first described for the identification of the DNA and RNA target sequences of nucleic-acid-binding proteins (1,2) but has since been used for the selection of the nucleic acid sequence ligands for other kinds of molecules (3). SELEX uses the power of genetic selection while taking advantage of in vitro biochemistry. It is a rapid technique that is relatively easy to implement and can accelerate and simplify nucleic-acid/protein interaction studies.