Dear Editor,Targeted gene knock-out and knock-in mice are valuable tools for elucidating the function of genes in vivo (Capecchi,2001).Recently,the Cas9 endonuclease from Streptococcus pyogenes type Ⅱ CRISPR system has been demonstrated as a powerful tool for gene targeting.Under the guidance of a synthetic 20-nucleotide single guide RNA (sgRNA),Cas9 protein can bind to specific genome locus and generate targeted double-stranded break (DSBs) to facilitate efficient genome editing (Cong et al.,2013;Mali et al.,2013).