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) (By similarity). The Cas4 region acts as a ssDNA exonuclease, while the Cas1 region acts as a dsDNA endonuclease. Involved in the integration of spacer DNA into the CRISPR cassette (By similarity). exonucleolytic cleavage in the 5'- to 3'-direction to yield nucleoside 3'-phosphates. Binds 1 [4Fe-4S] cluster per subunit. Homodimer, forms a heterotetramer with a Cas2 homodimer. In the N-terminal section; belongs to the CRISPR-associated exonuclease Cas4 family. In the C-terminal section; belongs to the CRISPR-associated endonuclease Cas1 family.