Technologies to achieve specific and precise genome editing, such as knock-in and knock-out, are critical for deciphering the functions of a gene and for understanding fundamental biological processes. Compared with the zinc finger nucleases (ZFN) and transcription activator-like effector nucleases (TALEN), which have been used for genome editing1, the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR-associated (Cas) system has emerged as a new powerful tool for genome modifications. It has recently been adopted for genome editing in human cell lines2,3,4, mouse5, zebrafish6, C. elegans7,8,9,10,11,12, and plants13.

In the widely used CRISPR/Cas9 system2,3,4, the Cas9 endonuclease is ushered to the specific site of interest by the single guide RNA (sgRNA), an engineered fusion molecule of the targeting CRISPR RNA (crRNA) with the trans-activating crRNA, to generate double-stranded DNA breaks (DSDBs) in the target site. The DSDBs can be repaired either through non-homologous end joining (NHEJ), which leads to generation of random deletions, insertions, or both (InDels)2,3,4,5,7,8,9,11,13, or through homologous recombination (HR), which could generate specific and precise nucleotide or sequence replacements3,5,9,10,12 when a plasmid or a single-stranded oligonucleotide (oligo) template is also present. The use of oligonucleotides as donor templates, which can be rapidly synthesized through commercial sources, to achieve Cas9-mediated knock-ins has not yet been reported in C. elegans.

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