sentences:  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.