CRISPR (clustered regularly interspaced short palindromic repeat) is an adaptive immune system that provides protection against mobile genetic elements (viruses, transposable elements and conjugative plasmids). CRISPR clusters contain spacers, sequences complementary to antecedent mobile elements, and target invading nucleic acids. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA). In type II CRISPR systems correct processing of pre-crRNA requires a trans-encoded small RNA (tracrRNA), endogenous ribonuclease 3 (rnc) and this protein, although RNase 3 is not required for 5'-processing of crRNA in this strain. Cas9/crRNA/tracrRNA endonucleolytically cleaves linear or circular dsDNA target complementary to the spacer; Cas9 is inactive in the absence of the 2 guide RNAs (gRNA, PubMed:23940360). Cas9 recognizes the protospacer adjacent motif (PAM) in the CRISPR repeat sequences to help distinguish self versus nonself, as targets within the bacterial CRISPR locus do not have PAMs. PAM recognition is also required for catalytic activity. Plasmids containing sequences homologous to endogenous spacer elements and that have flanking PAM consensus sequences cannot transform this strain unless the cas9 gene is disrupted or critical residues of Cas9 are mutated. Monomer. Binds crRNA and tracrRNA (Probable). Has 2 endonuclease domains. The discontinuous RuvC-like domain cleaves the target DNA noncomplementary to crRNA while the HNH nuclease domain cleaves the target DNA complementary to crRNA. Almost no mature crRNA or tracrRNA produced. Loss of CRISPR interference during plasmid transformation. Coexpression with both gRNAs or a synthetic gRNA in human embryonic or pluripotent stem cells shows it is possible to target and modify a DNA sequence of interest. Belongs to the CRISPR-associated protein Cas9 family. Subtype II-C subfamily.