🚀 go-pugleaf

There's lots of problems with a weapon which is a simple blocker of DNA
replication or transcription to RNA.  Even if you could find an agent
which a) can be absorbed through the skin or lungs b) is not destroyed
in the process c) spreads throughout the body, it would not be an
effective weapon because it would take weeks or months to incapacitate
the victim.  Not like nerve gas or phosgene, where they drop dead right
away.  Such an agent would share the other disadvantages of poison gas,
i.e. a) subject to shifts in the wind b) hard to keep off your own troops
c) easy to take countermeasures.

Now if you were talking about a nanotechnology robot poison gas, you might
have something.  These could be equipped with diamond cutting tools to
break through gas masks, they could have an "off" signal (such as a 
frequency of light) which could be used to decontaminate an area before
moving your own troops in, they might even produce specific changes in the
enemy such as rewiring their brains so that they're on your side.