CAS9 (CRISPR associated protein 9) is an RNA-guided DNA nuclease and part of the Streptococcus pyogenes CRISPR antiviral immunity system that provides adaptive immunity against extra chromosomal genetic material. The Cas9 protein and a guide RNA consisting of a fusion between a crRNA and a trans-activating crRNA (tracrRNA) must be introduced or expressed in a cell. A 20-nucleotide sequence at the 5' end of the guide RNA directs Cas9 to a specific DNA target site. As a result, Cas9 can be "programmed" to cut various DNA sites both in vitro and in cells and organisms. CRISPR/Cas9 genome editing tools have been used in many organisms, including mouse and human cells. Research studies demonstrate that CRISPR can be used to generate mutant alleles or reporter genes in rodents and primate embryonic stem cells.