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. In a heterologous host this Csm effector complex restricts ssRNA phage MS2, suggesting it may target RNA viruses in vivo. Csm functions as a non-specific ssDNase. Base-pairing between crRNA and target RNA to form a ternary Csm complex activates a ssDNase activity; target RNA cleavage suppresses the ssDNase, a temporal control that prevents uncontrolled DNA degradation. Viral RNA transcripts probably tether the Csm complex to the viral genome, recruiting Cas10 ssDNA activity which is able to degrade DNA in the transcription bubble, spatially controlling the DNase activity. This subunit might be involved in maturation of a crRNA intermediate to its mature form. Part of the Csm effector complex that includes at least Cas10(1), Csm2(3), Csm3(5), Csm4(1), Csm5(1) and mature crRNA (PubMed:25458845, PubMed:27105119, PubMed:28663439). The Csm complex is elongated and slightly twisted with a maximal length of 215 Angstroms and a diameter of 75-80 Angstroms (PubMed:25458845). It has been modeled to have a central protein filamant of Csm3 subunits along which the dsRNA helix of paired crRNA and target RNA binds. The filament is capped at one end by Cas10 and Csm4 and at the other end by Csm5; ssDNA is thought to bind to the N-terminal HD domain of Cas10 (Probable). Csm with a precursor crRNA does not include Csm5, while Cas6, the enzyme probably involved in pre-crRNA processing, is found associated with a subset of the Csm complex (PubMed:25458845). Encoded in a type III-A CRISPR locus. Belongs to the CRISPR-associated Csm5 family.