Dear Editor,CRISPR/Cas9 is a powerful tool for genome editing (Komor et al.,2017).Recently,it has been employed in several attempts to edit the human embryos (Liang et al.,2015;Kang et al.,2016;Tang et al.,2017).A major technical conce particularly relevant in studies involving human embryos is the potential off-target effects (Callaway,2016;Plaza Reyes and Lanner,2017).Consequently,development of safer genome editing strategy in human embryos is highly anticipated (Cyranoski and Reardon,2015).The offtarget mutation result in part from Cas9-mediated double strand break (DSB) of DNA.Recently,base editing (BE)without the introduction of DSB has been achieved.The key design for BE is to use a catalytically inactive Cas9 to recruit the cytidine deaminase APOBEC to target sequences,leading to conversion of C to T within a window of approximately five nucleotides (Komor et al.,2016).Therefore,BE is apparently determined by additional features of the target sequence and offers a potentially safer approach for genome editing.Here we report the initial technical assessment of applying BE3,base editor 3 (Komor et al.,2016),in discarded human tripronuclear embryos.