The process of enrichment is called SELEX. In a typical SELEX process, a random-sequence oligonucleotide library of 20�C100 nucleotides long is synthesized. The library normally contains Homatropine Bromide between 161013 and 161015 different oligonucleotide sequences. Only a few oligonucleotides out of this enormous pool strongly interact with the specific target and are therefore selected for enrichment. In principle, aptamers may incorporate the advantages of biological drugs, such as single chain monoclonal antibodies, and of small molecule drugs. On the one hand, aptamers show the affinity and specificity of monoclonal antibodies, while presenting the chemical versatility of synthetic drugs, which may be modified using medicinal chemistry and also produced on a large scale synthesis. To date, one aptamer has reached the market: Macugen, an anti-angiogenic drug for the treatment of neovascular age-related macular degeneration. Other aptamers are currently in clinical development, and many groups are developing antiviral aptamers. A typical aptamer is 5�C15 kDa in size, binds its target with high affinity and can discriminate among closely related targets. Structural studies have shown that aptamers are capable of using the same types of binding interactions that drive the affinity and specificity in antibody-antigen complexes. Despite the fact that oligonucleotides are formed by only four nucleotides, it has been proven that this is sufficient to obtain a variety of three-dimensional structures and to achieve chemical versatility comparable to that of proteins, forming specific binding with virtually any chemical compound. In general, it is necessary to have a 20% alteration of the original oligonucleotide sequence to switch any sequence from one secondary structure to another. Aptamers are known to be very sensitive to changes in sequence; minimal sequence alterations may harm both the affinity of an aptamer to its target, Carbimazole especially if the mutations are in the regions that are considered to be the binding site of the aptamer to its target molecule, or affect the secondary structure.