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 sequences complementary to antecedent mobile elements and target invading nucleic acids. Unlike many single-component effectors, this CRISPR-Cas system targets RNA (PubMed:28475872). CRISPR clusters are transcribed from pre-CRISPR RNA (crRNA) and processed into crRNA by this protein (PubMed:28475872). Cleaves linear target ssRNA in a pre-crRNA-dependent fashion, preferentially around A residues (PubMed:28475872). Binding a viable target RNA target activates this protein for non-specific RNA degradation in vitro (called collateral RNA degradation), but it is not very sensitive as it requires nanomolar levels of viable target RNA (PubMed:28475872). Pre-crRNA processing is metal independent, while crRNA-guided target RNA cleavage is dependent on divalent metal. Target RNA acts as an activator for non-specific ssRNA degradation (PubMed:28475872). The target ssRNase active sites are probably within the 2 HEPN-like folds, and the 2 folds interact in vivo. Part of a type VI-A adenine-prefering CRISPR-Cas system. Belongs to the CRISPR-associated endoribonuclease Cas13a family.