Cas9 (CRISPR associated protein 9) is an RNA-guided DNA endonuclease associated with the CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) type II adaptive immunity system in Streptococcus pyogenes, among other bacteria. Cas9 performs interrogation by unwinding foreign DNA and checking whether it is complementary to the 20 basepair spacer region of the guide RNA. If the DNA substrate is complementary to the guide RNA, Cas9 cleaves the invading DNA. The Cas9 protein has been heavily utilized as a genome engineering tool to induce site-directed double strand breaks in DNA. These breaks can lead to gene inactivation or the introduction of heterologous genes through non-homologous end joining and homologous recombination respectively in many laboratory model organisms.