So, just how mutable is the DNA? For the sake of the discussion, I don't know anything about DNA that I can say I didn't get from a sci-fi flick.
There's methyl groups that attach to the DNA. These stop parts of the DNA being read.
There's Histones which wind up your DNA and control how much can be read.
These are used control which parts of DNA are used by which cells. Evey cell has the same Genome, but each cell needs a different program to function, so changing the Gene Expression you get a different kind of cell.