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. Acts as a dsDNA and dsRNA endonuclease (in a fusion with Alicyclus caldarius esterase to increase solubility). Optimum pH is 3 and 7-8. Expressed as a fusion protein to increase stability. Optimum temperature is 50 degrees Celsius. Expressed as a fusion protein to increase stability. 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. Belongs to the CRISPR-associated nuclease Cas3-HD family.