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English: CRISPR-Cas9 as a Molecular Tool Introduces Targeted Double Strand DNA Breaks. By joining tracrRNA and crRNA derived sequences with a linker loop, a single guide RNA (sgRNA) is created. sgRNA can be synthesized chemically and thus makes it easy to use CRISPR-Cas9 as a programmable molecular tool. The complex consisting of sgRNA and Cas9 scans DNA for the presence of a protospacer adjacent motif (PAM). For Cas9 from S. pyogenes, this is a 5'-NGG-3' sequence. When a PAM sequence is detected, the complementary DNA strand is compared to the crRNA derived guide region. If these sequences match, the DNA double strand is cleaved ~3 bp away of the PAM, introducing a double-strand DNA break (DSB). With both domains located in the NUC lobe of Cas9, the HNH domain cuts the strand complementary to the guide sequence (target strand) while the RuvC domain cuts the opposite strand.