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). Functions as a ssRNA-specific endoribonuclease. Involved in the integration of spacer DNA into the CRISPR cassette (By similarity). Homodimer, forms a heterotetramer with a Cas1 homodimer (By similarity). Can form a Cascis complex with Cas4 and Csa1. Slightly induced by 20 J/m2 ultraviolet light. Member of the csa1-cas1/2-cas4 operon. In the N-terminal section; belongs to the CRISPR-associated endonuclease Cas1 family. In the C-terminal section; belongs to the CRISPR-associated endoribonuclease Cas2 protein family. Neither Cas1 nor Cas2 are fully present in this fusion protein, however as this is the only copy of these proteins in this organism they are probably functional. The Cas1 section is much smaller than usual and it is quite atypical.