Anti-Cas9 antibodies are useful reagents in the immunodetection of Cas 9 (CRISPR-associated protein 9) endonuclease, the enzyme that enables CRISPR-Cas9 gene editing. The S. pyogenes Cas9 is a large protein, featuring 1,368 amino acids in length with a mass of 158 kilodaltons. A mutated variant, Cas9 Endonuclease Dead (dCas9), contains point mutations in two residues that abolish the endonuclease activity.