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) (Probable). Functions as a ssRNA-specific endoribonuclease. Involved in the integration of spacer DNA into the CRISPR cassette (By similarity). When the CRISPR3/cas system consisting of cas9-cas1-cas2-csn2-CRISPR3 or just cas9-CRISPR3 is expressed in E.coli it prevents plasmids homologous to spacers 1 or 2 from transforming. Homodimer, forms a heterotetramer with a Cas1 homodimer. Plasmid transformation is still inhibited. Belongs to the CRISPR-associated endoribonuclease Cas2 protein family.