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 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 (By similarity). Homodimer, forms a heterotetramer with a Cas2 homodimer. Belongs to the CRISPR-associated endonuclease Cas1 family. Missing conserved residues thought to be important for metal-binding.