A protein derived from the CRISPR-Cas bacterial immune system that has been co-opted for genome engineering. Uses an RNA molecule as a guide to find a complementary DNA sequence. Once the target DNA is identified, Cas9 cuts both strands. Has been compared to “molecular scissors” or a “genetic scalpel.” In CRISPR immunity, cutting viral DNA prevents it from destroying the host cell. In genome engineering, cutting genomic DNA initiates a repair process that ends up making a change or “edit” to its sequence.