Clustered regularly interspaced short palindromic repeats(CRISPR)/CRISPR-associated protein 9(Cas9)-mediated genome editing can efficiently produce gene-knockout mutants.On the other hand,CRISPR/Cas-derived base editors offer the ability to induce precise nucleotide substitutions(Komor et al.,2016).Cytidine base editors(CBEs)consist of a cytidine deaminase fused with a Cas9-nickase variant(Cas9n,with a D10A substitu-tion)and can achieve site-specific C-to-T substitution.Similarly,adenine base editors use an adenine deaminase forA-to-G substi-tution.These systems have been used in various organisms(Mishra et al.,2019).However,the Cas9 complex requires target sites containing NGG protospacer adjacent motifs(PAMs),thus restricting selection of potential targets.A number of CBEs have been developed using Cas9 variants(mostly Cas9n),cytidine deaminases(such as rAPOBEC1 and PmCDA1),and uracil glycosylase inhibitor(UGI)domains.These CBEs of the first generation(BE1,rAPOBEC1-dCas9),second generation(BE2,rAPOBEC1-dCas9-UGI),and third generation(BE3,rAPOBEC1-Cas9n-UGI)have moderate editing efficiencies in mammalians(Komor etal.,2016).
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