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= Systematic Evolution of Ligands by Exponential Enrichment (SELEX) = Systematic evolution of ligands by exponential enrichment (SELEX), also referred to as in vitro selection or in vitro evolution, is a combinatorial chemistry technique in molecular biology for producing oligonucleotides of either single-stranded DNA or RNA, commonly referred to as aptamers, that specifically bind to a target ligand or ligands. Although SELEX has emerged as the most commonly used name for the procedure, some researchers have referred to it as SAAB (selected and amplified binding site) and CASTing (cyclic amplification and selection of targets) SELEX was first introduced in 1990. In 2015 a special issue was published in the Journal of Molecular Evolution in the honor of quarter century of the SELEX discovery.

The process begins with the synthesis of a very large oligonucleotide library consisting of randomly generated sequences of fixed length flanked by constant 5' and 3' ends that serve as primers. [Elington] For a randomly generated region of length n, the number of possible sequences in the library is 4n (n positions with four possibilities (A,T,C,G) at each position). The sequences in the library are exposed to the target ligand - which may be a protein or a small organic compound - and those that do not bind the target are removed, usually by affinity chromatography or target capture on paramagnetic beads (see if you can find a paper) .[ Cite ] The bound sequences are eluted and amplified by PCR [Elington] to prepare for subsequent rounds of selection in which the stringency of the elution conditions is increased to identify the tightest-binding sequences. An advancement on the original method allows an RNA library to omit the constant primer regions, which can be difficult to remove after the selection process because they stabilize secondary structures that are unstable when formed by the random region alone.

SELEX has been used to develop a number of aptamers that bind targets interesting for both clinical and research purposes. Towards these ends, a number of nucleotides with chemically modified sugars and bases have been incorporated into SELEX reactions. [Cite] These modified nucleotides allow for the selection of aptamers with novel binding properties and potentially improved stability. [Cite]

Generating Single Stranded Oligonucleotide Library
Chemical synthesis of randomized oligonucleotide sequences and PCR amplification to ensure replication capability and associated loss of diversity [Elington]

Target Incubation and Binding Sequence Amplification
Distinct target immobilization techniques, considerations with time, temperature, conditions of incubation, etc.

Obtaining ssDNA
One of the most critical steps in the SELEX procedure is obtaining single stranded DNA (ssDNA) after the PCR amplification step. This will serve as input for the next cycle so it is of vital importance that all the DNA is single stranded and as little as possible is lost. Because of the relative simplicity, one of the most used methods is using biotinylated reverse primers in the amplification step, after which the complementary strands can be bound to a resin followed by elution of the other strand with lye. Another method is asymmetric PCR, where the amplification step is performed with an excess of forward primer and very little reverse primer, which leads to the production of more of the desired strand. A drawback of this method is that the product should be purified from double stranded DNA (dsDNA) and other left-over material from the PCR reaction. Enzymatic degradation of the unwanted strand can be performed by tagging this strand using a phosphate-probed primer, as it is recognized by enzymes such as Lambda exonuclease. These enzymes then selectively degrade the phosphate tagged strand leaving the complementary strand intact. All of these methods recover approximately 50 to 70% of the DNA. For a detailed comparison refer to the article by Svobodová et al. where these, and other, methods are experimentally compared.

Further Caveats and Considerations
A caution to consider in this method is that the selection of extremely high, sub-nanomolar binding affinity entities may not in fact improve specificity for the target molecule. Off-target binding to related molecules could have significant clinical effects. [Cite?]

Prior SELEX Targets (new)
The technique has been used to evolve aptamers of extremely high binding affinity to a variety of target ligands, including small molecules such as ATP and adenosine and proteins such as prions and vascular endothelial growth factor (VEGF). Moreover, SELEX has been used to select high-affinity aptamers for complex targets such as tumor cells. Clinical uses of the technique are suggested by aptamers that bind tumor markers, GFP-related fluorophores, and a VEGF-binding aptamer trade-named Macugen has been approved by the FDA for treatment of macular degeneration. Additionally, SELEX has been utilized to obtain highly specific catalytic DNA or DNAzymes. Several metal-specific DNAzymes have been reported including the GR-5 DNAzyme (lead-specific), the CA1-3 DNAzymes (copper-specific), the 39E DNAzyme (uranyl-specific) and the NaA43 DNAzyme (sodium-specific).

Chemically Modified Nucleotides in SELEX (new)
Why use these modified bases?: Diversity, hydrophilic and hydrophobic properties, nuclease resistance, etc.

The genetic alphabet, and thus possible aptamers, is expanded using unnatural base pairs was applied to SELEX and high affinity DNA aptamers were generated.