Challenges to increasing targeting efficiency in genome engineering

Gene targeting technologies are essential for the analysis of gene functions. Knockout mouse generation via genetic modification of embryonic stem cells (ESCs) is the commonest example, but it is a time-consuming and labor-intensive procedure. Recently, a novel genome editing technology called CRISPR/Cas has enabled the direct production of knockout mice by non-homologous end joining (NHEJ)-mediated mutations. Unexpectedly, however, it generally exhibits a low efficiency in homologous recombination (HR) and is prone to high mosaicism. Meanwhile, gene targeting using ESCs is still being improved, as reported by Fukuda et al. in this issue. Here, we outline current gene targeting technologies with special emphasis on HR-mediated technologies, which are currently being performed using these two major strategies.