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) (By similarity). Sometimes seen associated with the aCascade ribonucleoprotein complex, minimally composed of Csa2 and Cas5a, which binds crRNA. Other probable components of aCascade in strain P1 are Cas6 and Csa5, while SSO1399, Cas5b (SSO1400) and SSO1401 have sometimes been seen weakly associated. The Csa2-Cas5a-crRNA complex also binds target DNA homologous to crRNA, probably forming an R-loop. Purified aCascade forms a filament about 6 nm in width. The aCascade complex was purified from strain P1.