SELEX is a combinatorial chemistry technique for producing aptamers that specifically bind to a target ligand.

Since SELEX was first introduced in 1990 by Dr. Larry Gold at the University of Colorado, various aptamers have been developed that bind different targets. The aptamers are selected through an iterative process consisting of binding the target with a highly diverse oligonucleotides library, washing unbound aptamer and amplification of survived oligonucleotides. Multiple rounds of SELEX enhance the enrichment of the oligonucleotides with high specificity for their targets. The best aptamer clones are selected after sequencing analysis and affinity assay with enriched oligonucleotides.