Using the SELEX Combinatorial Chemistry Process to Find High Affinity Nucleic Acid Ligands to Target Molecules

The SELEX (S ystematic E volution of L igands by EX ponential enrichments) combinatorial chemistry process is a procedure by which nucleic acid ligands of high affinity can be isolated against a molecular target (1 ). A wide variety of target molecules have been used successfully in the SELEX process, including nucleic acid binding proteins (1 –19 ), nucleic acid enzymes (1 ,20 –25 ), proteins with no known function involving nucleic acids (26 –31 ), a peptide (32 ), an RNA hairpin (33 ), and small molecules, such as organic dyes (34 ), amino acids (35 ,36 ), cyanocobalamin (37 ), theophylline (38 ), and ATP (39 ,40 ). The procedure involves:

1.	The synthesis of a template containing a variable region flanked by 5′ and 3′ fixed regions that enable amplification by PCR,
2.	Conversion of the template to an amplified library of candidate ligands,
3.	A binding reaction in which a fraction of the candidate ligands are bound by the target molecule;
4.	Separation of target molecule/ligand complexes from unbound candidate ligands;
5.	Purification of the nucleic acid ligands from the separated complexes; and
6.	Replication of the nucleic acid ligand population.