Targeted genome editing using engineered nucleases has attracted attention in the field of gene function analysis and gene therapy. Although ZFN and TALEN fusion proteins that connect the FokI endonuclease with DNA-binding domains have been developed and used; the labor-intensive and time-consuming process of the construction of a DNA-binding domain has hindered the spread of their use. Emergence of the clustered, regularly interspaced, short palindromic repeat (CRISPR)/Cas system has changed this situation. This system is far simpler to prepare because of the use of guide RNA (gRNA) to direct the CAS9 enzyme to the target loci--the gRNA can be easily prepared by changing 20 nucleotides of the 5′ region of the gRNA. Thus a prepared, CRISPR/Cas system efficiently works in mammalian cells. Moreover, gene knockout mice can be generated within a month by injecting RNAs or plasmids coding gRNA and Cas9 into fertilized eggs. Here we introduce our simple evaluation system for gRNAs and our plasmid injection system for generating gene-knockout mice. Further applications of the CRISPR/Cas system for point mutation, large deletion, reporter gene knockin, and other advances to the technique, will also be reviewed.