The bacterium then transcribes the CRISPR DNA and produces two specialized types of RNA: CRISPR RNA (crRNA), which matches the viral DNA, and tracer RNA (Step 3). Both of those RNA pieces then form a complex with CAS9, an endonuclease, or an enzyme that can cut DNA (Step 4). The guide crRNA, now part of the CAS complex, finds the matching sequence on the viral DNA (Step 5). The CAS protein-RNA complex unwinds and cuts the viral DNA (Step 6). Because the viral DNA is cut, it can no longer infect the bacterial cell.