A) Schematic representation of a single guide RNA, a synthetic RNA molecule resulting from the fusion of crRNA (N20, red) and the scaffolding tracrRNA (black). sgRNAs are required and sufficient to target the endonuclease Cas9 to specific sequences in the genome. B) Schematic representation of a target DNA sequence which comprises the protospacer element (N20, black) upstream of the protospacer adjacent motif (PAM, NGG, blue). Arrows indicate the location of the double strand break introduce by the Cas9 endonuclease.