Natural system used by bacteria to protect themselves from infection by viruses when bacteria detect the virus dna, 2 rna strands are produced 1 of which contains a sequence that is the same as the invading virus the 2 rna strands form a complex with cas-9 cas-9 is a nuclease (enzyme that can cut dna) when the matching sequence, guiding dna, finds the target within the viral genome Cas9 cuts the dna and disables the virus by changing the guide rna to a desired target, the cas9 can precisely cut any sequence not just virus