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. CRISPR clusters are transcribed and processed into CRISPR RNA (crRNA). Cas3 plus Cascade participate in CRISPR interference, the third stage of CRISPR immunity. Anneals and unwinds R-loops (in which crRNA binds the target DNA, displacing the noncomplementary strand). Unwinding requires ATP, annealing does not. Proteins of this family have an N-terminal nuclease domain and a C-terminal helicase/ATPase domain. In some CRISPR/Cas systems the domains are swapped, in others they are encoded separately. In the N-terminal section; belongs to the CRISPR-associated nuclease Cas3-HD family. In the central section; belongs to the CRISPR-associated helicase Cas3 family.