The mark doesn't just tell officers that the person was hit with the gun. It can also tell them exactly where she was and when. That's because the pellets in each gun have a specific DNA sequence, matched to the officer it is issued to. Since the firing is authorized by a superior, it's easy to tell when the suspect picked up the mark.
Combined with evidence from local video cameras and witnesses it provides an ironclad case for when a given person was at the scene of a crime, said Knights.
"It closes the evidentiary triangle," Knights said, referring to the time, place, and the person involved in an incident. "They can't say 'it wasn't me, just some bloke who looks like me.'"
The two companies use slightly different types of DNA. Applied DNA uses a two-stranded set of sequences derived from a plant, though the sequence itself is completely artificial. SelectaMark uses single-stranded DNA. (Neither set can reproduce, nor are they functional).
Such tags are hard to copy. DNA is made of four nucleotides, which make up the code that geneticists read. In living things those nucleotides (marked A, G, C, and T) that make base pairs, and they can only occur in certain combinations because the DNA has to code for proteins. But artificial DNA doesn't have that constraint. A single strand of DNA just 20 base pairs long gives 1.1 trillion possible combinations.