Dear Editor,The newly developed CRISPR/Cas9-mediated base editing technology with cytosine deaminase is capable of precisely and efficiently introducing point mutations at the target genomic locus,which does not require double-stranded DNA breaks or any donor templates and thus exhibit a great potential for gene correction and genetic diversification in yeasts,plants,and mammalian and human cells (Komor et al.,2016;Nishida et al.,2016;Lu and Zhu,2017;Ren et al.,2017).However,compared with AID/APOBEC1 members of the cytosine deaminase family that are widely utilized in base editing to induce cytidine (C) to thymine (T) conversion,adenosine deaminase is far from being applicable since TadA/ADAR members act strictly on duplex RNA,or DNA/RNA hybrids with mismatches,instead of single-stranded DNA (Zheng et al.,2017).To address this problem,great efforts have been invested recently in identifying Escherichia coli TadA variants that accept DNA as a substrate through rounds of protein evolution and engineering,ultimately leading to a number of adenine base editors (ABEs) with great efficiencies and broadened sequence compatibility in inducing nucleotide changes at a wide range of target genomic loci in human cells (Gaudelli et al.,2017).In these ABE systems,the TadA:TadA* heterodimer is guided by the Cas9n/single guide RNA (sgRNA) complex to the target site,and the engineered TadA*,but not the wild-type TadA,functions as the active tRNA adenosine deaminase that tus adenine (A) to inosine (Ⅰ) in singlestranded genomic DNA,subsequently resulting in A to guanine (G) mutation in genome during DNA repair or DNA replication (Gaudelli et al.,2017).These tools,together with previous base editors,enable programmable introduction of all four transitions (C to T,G to A,A to G,and T to C) at the target loci in the genome,greatly expanding the capabilities of base editing.Here,we report the development of fluorescencetracking base editing systems with E.coil TadA variants and Cas9 variants in rice.