are highly selective for a certain cofactor. Among this group are the metal selective deoxyribozymes such as Pb2+-specific 17E, UO22+-specific 39E, and Na+-specific A43. First crystal structure of a DNAzyme was reported in 2016. 10-23 core based DNAzymes and the respective MNAzymes that catalyse reactions at ambient temperatures were described in 2018 and open doors for use of these nucleic acid based enzymes for many other applications without the need for heating. This link and this link describe the DNA molecule 5'-GGAGAACGCGAGGCAAGGCTGGGAGAAATGTGGATCACGATT-3' , which acts as a deoxyribozyme that uses light to repair a thymine dimer, using serotonin as cofactor. RNA ligases Of particular interest are DNA ligases. These molecules have demonstrated remarkable chemoselectivity in RNA branching reactions. Although each repeating unit in a RNA strand owns a free hydroxyl group, the DNA ligase takes just one of them as a branching starting point. This cannot be done with traditional organic chemistry. Other reactions Many other deoxyribozymes have since been developed that catalyze DNA phosphorylation, DNA adenylation, DNA deglycosylation, porphyrin metalation, thymine dimer photoreversion and DNA cleavage. Methods in vitro selection Because there are no known naturally occurring deoxyribozymes, most known deoxyribozyme sequences have been discovered through a high-throughput in vitro selection technique, similar to SELEX. in vitro selection utilizes a "pool" of a large number of random DNA sequences (typically 1014–1015 unique strands) that can be screened for a specific catalytic activity. The pool is synthesized through solid phase synthesis such that each strand has two constant regions (primer binding sites for PCR amplification) flanking a random region of a certain length, typically 25–50 bases long. Thus the total number of unique strands, called the sequence space, is 4N where N denotes the number of bases in the random region. Because 425 ≈ 1015, there is no practical reason to choose random regions of less than 25 bases in length, while going above this number of bases means that the total sequence space cannot be surveyed. However, since there are likely many potential candidates for a given catalytic reaction within the sequence space, random regions of 50 and even higher have successfully yielded catalytic deoxyribozymes. The pool is first subjected to a selection step, during which the catalytic strands are separated from the non-catalytic strands. The exact separation method will depend on the reaction being catalyzed. As an example, the separation step for ribonucleotide cleavage often utilizes affinity chromatography,