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, generating an 8 base-long tag known as the 5' handle. Possibly part of the aCascade ribonucleoprotein complex. Belongs to the CRISPR-associated endoribonuclease Cas6 family.