[Recent developments in enhancing the efficiency of CRISPR/Cas9- mediated knock-in in animals]

Gene-editing technology can artificially modify genetic material of targeted loci by precise insertion, deletion, or replacement in the genomic DNA. In recent years, with the developments of zinc-finger endonuclease (ZFN), transcription activator-like effector nuclease (TALEN), clustered regularly interspaced short palindromic repeats/CRISPR- associated protein 9 (CRISPR/Cas9) technologies, such precise modifications of the animal genomes have become possible. Although gene-editing tools, such as CRISPR/Cas9, can efficiently generate double-strand breaks (DSBs) in mammalian cells, the homology-directed repair (HDR) mediated knock-in (KI) efficiency is extremely low. In this review, we briefly describe the current development of gene-editing tools and summarize the recent strategies to enhance the CRISPR/Cas9- mediated KI efficiency, which will provide a reference for the generation of human disease models, research on gene therapy and livestock genetic improvement.