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). The type III-A Csm effector complex binds crRNA and acts as a crRNA-guided RNase, DNase and cyclic oligoadenylate synthase; binding of target RNA cognate to the crRNA is required for all activities. The subunit probably binds to the 5' handle of the crRNA, helping in discrimination between self- and non-self. Probably part of the Csm effector complex, that includes Cas10, Csm2, Csm3, Csm4, Csm5 and mature crRNA (By similarity). Interacts with Cas10 (csm1) (PubMed:25773141). Encoded in a type III-A CRISPR locus. Belongs to the CRISPR-associated Csm4 family.