[Genome editing technology: from basics to applications].

The CRISPR/Cas9 system was initially discovered as a means of acquired immune response in bacterial species and has been developed and applied to genome editing technology in mammalian cells. This system consists of three key components: crRNA, tracrRNA, and Cas9 protein. Once Cas9 is drawn to the target sequence, it creates DNA double-strand breaks, which then undergo repair via nonhomologous end joining or homology-directed repair. Thus, the CRISPR/Cas9 system enables us to knock out the gene of interest and insert the desired sequences for downstream analyses and clinical applications. Because of the simplicity of CRISPR/Cas9 technology, it has been widely adopted. For effective genome editing, several factors such as off-target effect and CRISPR/Cas9 delivery methods should be considered. Beyond gene knockout and nucleotide substitutions, CRISPR/Cas9 has been applied for various purposes, including more flexible nucleotide substitutions, transcriptional regulation, epigenetic modification, chromatin-chromatin interaction, and live-cell imaging using the nuclease domain deactivated mutant Cas9s, nCas9 and dCas9. This chapter discusses the expanding CRISPR/Cas9 technology-from basics to applications.