Cas9 (CRISPR-associated protein 9, formerly Cas5, Csn1, or Csx12) is a DNA cutting enzyme that is part of the CRISPR immune system in bacteria and archaea. It has been adapted to be used as a CRISPR genome editing tool that enables targeted modification of DNA sequences and permanently changes a cell’s genome.
The CRISPR-Cas10 system is widely used in genetic engineering, medicine, agriculture, and biotechnology. Cas9 introduces site-specific double-stranded breaks in DNA, enabling gene inactivation, removal, or insertion through cellular repair mechanisms such as non-homologous end joining or homologous recombination. Variants of Cas9, including Cas9 nickase (Cas9n), that create single-strand breaks, and versions recognizing different PAM sequences, have been developed to address limitations in CRISPR-Cas9 editing.
The development of CRISPR-Cas9 technology has raised ethical and regulatory concerns, particularly regarding human germline editing and genetic enhancement.