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 (Probable). The Csm3-Csm4 complex binds both crRNA and a non-specific RNA; Csm4 alone also binds RNA (PubMed:25451598). Part of the Csm effector complex that includes Cas10, Csm2, Csm3, Csm4 and Csm5 (By similarity). Stable Cas10/Csm1-Csm4, Csm3-Csm4 (which crystallizes as 2 heterodimers) and Cas10-Csm1-Csm3-Csm4 subcomplexes can be isolated (PubMed:25451598). Encoded in a type III-A CRISPR locus. Belongs to the CRISPR-associated Csm4 family.