FIGURE 1.

The principle of CRISPR/Cas9 mediated genome editing and criteria for guide RNA selection. (A) In the CRISPR/Cas9 system, a 20 nt guide RNA (gRNA) is complementary to the target DNA region in the host genome followed by a gRNA scaffold sequence. Each target DNA sequence ends with a protospacer adjacent motif (PAM), which is often the sequence “NGG.” The formation of the gRNA-DNA complex triggers the binding of the Cas9 endonuclease to the complex and generates a double-stranded break (DSB) 3 bp in front of the PAM. (B) General rules for choosing a gRNA sequence to improve its effectiveness.