Immediately prior to target introduction, the single stranded oligonucleotide library is often heated and cooled slowly to renature oligonucleotides into thermodynamically stable secondary and tertiary structures. Once prepared, the randomized library is incubated with immobilized target to allow oligonucleotide-target binding. There are several considerations for this target incubation step, including the target immobilization method and strategies for subsequent unbound oligonucleotide separation, incubation time and temperature, incubation buffer conditions, and target versus oligonucleotide concentrations. Examples of target immobilization methods include affinity chromatography columns, nitrocellulose binding assay filters, and paramagnetic beads. Recently, SELEX reactions have been developed where the target is whole cells, which are expanded near complete confluence and incubated with the oligonucleotide library on culture plates. Incubation buffer conditions are altered based on the intended target and desired function of the selected aptamer. For example, in the case of negatively charged small molecules and proteins, high salt buffers are used for charge screening to allow nucleotides to approach the target and increase the chance of a specific binding event. Alternatively, if the desired aptamer function is in vivo protein or whole cell binding for potential therapeutic or diagnostic application, incubation buffer conditions similar to in vivo plasma salt concentrations and homeostatic temperatures are more likely to generate aptamers that can bind in vivo. Another consideration in incubation buffer conditions is non-specific competitors. If there is a high likelihood of non-specific oligonucleotide retention in the reaction conditions, non specific competitors, which are small molecules or polymers other than the SELEX library that have similar physical properties to the library oligonucleotides, can be used to occupy these non-specific binding sites. Varying the relative concentration of target and oligonucleotides can also affect properties of the selected aptamers. If a good binding affinity for the selected aptamer is not a concern, then an excess of target can be used to increase the probability that at least some sequences will bind during incubation and be retained. However, this provides no selective pressure for high binding affinity, which requires the oligonucleotide library to be in excess so that there is competition between unique sequences for available specific binding sites.