Precise modification of eukaryotic genomes has been accomplished mainly through homology-directed repair (HDR) of DNA double-strand breaks (DSBs) (Hess et al., 2017). However, the inherent low efficiency of homologous recombination and poor availability of exogenous donor DNA as repair templates strongly impede the use of HDR for precise genome editing in many species (Komor et al., 2017a). To complement the HDR method and circumvent some of its limitations, the recently developed base editing approach has enabled irreversible conversion of cytidine (C) to thymidine (T) (or guanine [G] to adenine [A]) at target loci without requiring DSB formation and HDR (Komor et al., 2016, Nishida et al., 2016). The most efficient base editor, BE3, consists of the cytidine deaminase APOBEC1 fused with a Cas9 nickase (nCas9(D10A)) and the uracil glycosylase inhibitor UGI that manipulates the DNA repair pathway (Komor et al., 2016). BE3 efficiently converts C·G to T·A in a programmable manner in a wide range of species including plants (Hess et al., 2017, Li et al., 2017, Lu and Zhu, 2017). This technology was improved significantly through engineering the cytidine deaminase and adding the bacteriophage Mu protein (Kim et al., 2017, Komor et al., 2017b). Despite the high efficiency and precision of the cytidine deaminase-mediated C–T conversion, additional base editing tools are needed for increasing the versatility of the base editing technology.

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