DNA bio-weapon that can target a single person

This was posted, and flagged, earlier today at https://news.ycombinator.com/item?id=32210961 .

My comment there was:

Might make for a good movie, but otherwise just a bunch of malarkey.

There isn't a short set of DNA which marks each person uniquely. This sci-fi weapon needs to recognize a scattered set of changes in the DNA, then activate some sort of toxin? Or trigger a virus? Which must be enough to kill the person, but not affect anyone else.

So it's what, a bioweapon - with all the difficulties that entails - then a sort of sensor genetically added to the viral/bacterial genome so each cell can recognize just the right DNA, along with some sort of error correction system to prevent mutations in that sensor system.

And, you know what you could have if you could create genomically targeted bioweapons? A cure for cancer.