According to this fascinating and scary article, in 1997, researcher Se-Jin Lee published work showing that inactivating a certain gene in mice led to insane muscle growth (twice the muscle mass!) in a very short amount of time. He was instantly inundated with requests from athletes, and in one case even a high school wrestling coach offering an entire team wrestlers as experiment subjects. Blood and urine test won't catch this kind of performance enhancement because it's not a supplement--it's an alteration of your genes.
Now a decade later, and you have to wonder: how many labs on this planet can perform these tricks? And is there any chance no top athletes are partaking?