Engineering of human immune cells to fight off cancer promised to help cure the disease, and, so far, it appears to be living up to the hype. So you might think that enlisting patients to take part in trials of the new therapies would be easy. But the New York Times reports that there's a hitch. First, many biotech firms are creating similar and competing treatments. Second, they can't be offered to all patients, perhaps because their cancers are treated surgically or because they don't attend a hospital involved with research. Combined, those factors mean that many trials are failing to recruit enough people to obtain useful results—and it's even worse for targeted therapies.