cleaves double stranded DNA at specifiable sequences

The protein, Cas9 (CRISPR Associated protein 9) is an anti-viral protein found in Streptococcus pyrogenes (of ‘Strep’ throat fame). The protein accepts a guide-RNA that it can use to match against DNA with the same sequence. When it arrives at DNA containing the same sequence as found in its guide RNA, Cas9 will cut the DNA at that location. For this homing capability it has found enormous use. The ability to seek out particular DNA strands for subsequent modification is useful if you wish to edit the genome of an organism - either by removing, adding, or modifying an existing sequence.

There are a number of other organisms that have variants of the Cas9 found in Streptococcus pyogenes. Functional Cas9 variants have been found in Candidatus , Campylobacter Jejuni , Neisseria meningitidis , Staphylococcus aureus Jejuni , and more. And a number of synthetic constructs have been built upon those natural proteins: a transcriptional activator , an epigenetic modifier , an optogentic Cas9 that is activated by light, etc.