**2.3 Base editors (BE)**

All programmable nucleases generate DSBs that are repaired by either NHEJ or HDR [7–10]. The common disadvantage of these nucleases is the non-predictable results of DSB repairing due to NHEJ mechanism and the low efficacy of HDR [13]. In this reason it became necessary to develop new methods for the introduction of point mutations excluding of DSBs, which resulted in GE new tools so-called base editors (BE) [38].

BE technologies use nickase Cas9 (nCas9) or functionally inactive Cas9 (dCas9) combined with a cytosine or adenosine deaminase domain, which bring into action transformation bases [39]. For example, cytosine deaminases transform cytosine (C) to uracil (U), which is identified as thymine (T) during following DNA repair and replication, thus providing a C:G to T:A replacement [38, 39]. In the same way, adenine deaminases convert adenine (A) to inosine (I), which polymerases interpreting as guanine (G), creating A:T for G:C replacement [39]. Cytidine deaminasebased BE (CBE) has been used to edit genomes in rice, *Arabidopsis*, wheat, corn, tomato and watermelon [39–41].

In the lack of known adenine deaminases, the substitution of A:T to G:C is a more difficult task than on the contrary [39]. This problem has been solved by bio-engineering of tRNA adenosine deaminases for their adaptation to DNA as a substrate [39–41].

Initially developed BE had an editor window of for several base pairs only, that led to the appearance of unexpected random mutations. But further improvement of these methods allows to create high-precision Cas9-based BEs that control the length and flexibility of the linker and thus able to selectively edit the bases in direct position with high accuracy, efficiency and simplified PAM requirements [39–41].
