In the July 2004 Discover Magazine there's an article about gene therapy, specifically the use of IGF-I, an insulin like growth hormone that drives muscle and growth repair. It's original intent was to help the disabled, sick and injured get better. Apparently the therapy has shown substatial strength gains in lab mice. A 15% increase in less active mice and a 30% increase in very active mice over a two month time frame. It also repairs muscle tissue remarkably fast
The doctor who has been heading up the research stated this therapy is very difficult to detect and only a muscle biopsy could do that, something no world class athlete would allow themselves to be subjected to. It has not been approved for humans yet but he says he's sure someone out there is preparing to do just that. The technology is fairly simple, at graduate school level, so anyone with the knowledge could develop the serum.
Of course the doctor warns that this is still potentially dangerous but so are steroids. And as the article states, "Nobody ever said athletes are the smartest people in the world" so they expect them to willingly take the risk.
I'm curious to see how this will pan out.
