A while back, I had a question from a reader about the stories you hear on the news: "New Genetic Discovery Could Lead to Therapy For Disease X". Just how large is the gap between DNA and a drug? The short answer: mighty spacious. Once in a while, you're set up for a short cut. But to balance those out, in the majority of cases you can't get there at all. There are all kinds of ways the process can go off track.
At the genetic level, it's unusual for a single defect to cause an entire disease. There are all sorts of rare conditions caused by such mutations, but most of these are known through only a handful of examples. People have searched the genome back and forth for genes linked to, say, Type II diabetes or schizophrenia, but those diseases just aren't that simple. As is also the case with many types of cancer, there seems to be a constellation of genetic factors that can make you more or less susceptible to the disease, but there's not going to be one single cause. That makes drug therapy hard, because we need those important bottlenecks and pivot points to aim our molecules at.
Even if you find one of those, the next big question is what kind of protein has been implicated from the genetic study. Medicinal chemistry can do pretty well with inhibition of enzymes or blockade of receptors, since those pathways just involve gumming up the works somewhere. Still, there are still large classes of enzymes with no good inhibitors (phosphatases!) and finding small molecules for big protein-liganded receptors isn't easy, either.
But if you need gain-of-function to treat the disease, well, you're almost always out of luck. There's often no handle for a drug to hit to make things work better. For a receptor, we could try to find more and more potent agonists, but a genetic problem with a receptor usually means that it's not signaling properly in the first place. Shouting louder into the phone doesn't work when the connection is broken.
A third level of difficulty is that many disease pathways don't have an obvious place for small molecules at all. Receptors and enzymes have binding pockets already built into them, which we can try to exploit. But there are untold zillions of protein-protein and protein-DNA interactions, and getting small molecules to work on those is a real challenge. It's actually been done, but you can count the successful examples on your fingers. (And if by successful you mean "on the market", I'm not even sure if you need any fingers at all. . .) My usual analogy is that it's like trying to keep two battleships from banging together by sticking a rowboat in between them.
Those levels of difficulty don't leave you very much to work with. If everything is lined up just right, you can take your chances with drug development the way it usually goes - otherwise, it's even harder. A huge amount of money has been spent over the last five to ten years in the drug business, digging through the genome for new targets. It's safe to say that most of that expenditure is a sunk cost on the level of, say, the Andrea Doria.