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). Involved in the integration of spacer DNA into the CRISPR cassette (By similarity). Functions as a ssRNA-specific endoribonuclease. Homodimer, forms a heterotetramer with a Cas1 homodimer. Belongs to the CRISPR-associated endoribonuclease Cas2 protein family.