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). 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 the appropriately targeted Csm effector complex prevents growth of dsDNA phage phiNM1-gamma6. This protein is not part of the Csm effector complex (PubMed:28722012). A single-strand-specific endoribonuclease (ssRNase) (PubMed:28722012). Activity is stimulated by cyclic oligoadenylates (cOA); maximal stimulation is seen with cyclic hexaadenylate (cA6) (PubMed:28722012). Non-specific ssRNase activity is allosterically activated by cyclic hexaadenylate (cA6) binding to its CARF domain. cA6 is a second messenger produced by Cas10 of the ternary Csm effector complex in the presence of a cognate target RNA. Homodimer; the composite ssRNase active site is formed at the dimer interface. The N-terminal CRISPR-associated Rossman fold (CARF) probably binds the cA6 effector. ssRNase activity resides in the C-terminal HEPN domain. Encoded in a type III-A CRISPR locus. Belongs to the CRISPR-associated Csm6 family.