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). Acts as a dsDNA endonuclease. Involved in the integration of spacer DNA into the CRISPR cassette. In vitro catalyzes a concerted transesterification reaction on branched DNA, as would be expected during integration of protospacers into the CRISPR leader sequence; Cas2 is not required in vitro. This reaction requires a 3'-OH group at the branch point (PubMed:26284603). Binds ss- and dsDNA and ss- and dsRNA with approximately equal affinity. May be able to anneal complementary DNA strands (PubMed:19427858). Homodimer, forms a heterotetramer with a Cas2 homodimer (By similarity). Forms oligomers, probably binds nucleic acids as a homodimer (PubMed:19427858). Belongs to the CRISPR-associated endonuclease Cas1 family.