Differences in individual patients have a major impact on how people react to drugs, making it extremely difficult for drug companies to design effective clinical trials, writes Clifton Leaf. "When you do any kind of trial, you're really trying to answer a question about truth in the universe," says Hal Barron of Roche Holding and Genentech. "And, of course, we can't know that. So we try to design an experiment on a subpopulation of the world that we think is generalizable to the overall universe." Designing small clinical studies and enrolling only those with suitable genetic or molecular profiles is a possible solution, but this approach could prove difficult, Leaf writes, suggesting instead a rework of the clinical trials themselves.