SELEX法（Systematic Evolution of Ligands by Exponential Enrichment）とは特定の分子標的に強く結合する核酸アプタマーを見いだすための手法。1990年代に確立された。抗体などと異なり生物を使用せず、試験管内で標的化合物を取得できることから試験管内選択法（in vitro selection）とも言う。
Tuerk, C.; Gold, L. Science 1990, 249, 505. DOI:10.1126/science.2200121
High-affinity nucleic acid ligands for a protein were isolated by a procedure that depends on alternate cycles of ligand selection from pools of variant sequences and amplification of the bound species. Multiple rounds exponentially enrich the population for the highest affinity species that can be clonally isolated and characterized. In particular one eight-base region of an RNA that interacts with the T4 DNA polymerase was chosen and randomized. Two different sequences were selected by this procedure from the calculated pool of 65,536 species. One is the wild-type sequence found in the bacteriophage mRNA; one is varied from wild type at four positions. The binding constants of these two RNA’s to T4 DNA polymerase are equivalent. These protocols with minimal modification can yield high-affinity ligands for any protein that binds nucleic acids as part of its function; high-affinity ligands could conceivably be developed for any target molecule.
(b) “In vitro selection of RNA molecules that bind specific ligands”
Ellington, A. D.; Szostak, J. W. Nature?1990, 346, 818. doi:10.1038/346818a0
 review:”SELEX—A (r)evolutionary method to generate high-affinity nucleic acid ligands”
Subpopulations of RNA molecules that bind specifically to a variety of organic dyes have been isolated from a population of random sequence RNA molecules. Roughly one in 1010 random sequence RNA molecules folds in such a way as to create a specific binding site for small ligands.
Stoltenburg, R.; Reinemann, C.; Strehlitz, B. Biomol. Eng. 2007, 24, 381. doi:10.1016/j.bioeng.2007.06.001
SELEX stands for systematic evolution of ligands by exponential enrichment. This method, described primarily in 1990 [Ellington, A.D., Szostak, J.W., 1990. In vitro selection of RNA molecules that bind specific ligands. Nature 346, 818–822; Tuerk, C., Gold, L., 1990. Systematic evolution of ligands by exponential enrichment: RNA ligands to bacteriophage T4 DNA polymerase. Science 249, 505–510] aims at the development of aptamers, which are oligonucleotides (RNA or ssDNA) binding to their target with high selectivity and sensitivity because of their three-dimensional shape. Aptamers are all new ligands with a high affinity for considerably differing molecules ranging from large targets as proteins over peptides, complex molecules to drugs and organic small molecules or even metal ions. Aptamers are widely used, including medical and pharmaceutical basic research, drug development, diagnosis, and therapy. Analytical and separation tools bearing aptamers as molecular recognition and binding elements are another big field of application. Moreover, aptamers are used for the investigation of binding phenomena in proteomics. The SELEX method was modified over the years in different ways to become more efficient and less time consuming, to reach higher affinities of the aptamers selected and for automation of the process. This review is focused on the development of aptamers by use of SELEX and gives an overview about technologies, advantages, limitations, and applications of aptamers.