The Gatekeeper

In 1999, French investigators undertook a daring experiment. Their patients were eleven children with a devastating immune system disorder called X-SCID, popularly known as "bubble boy" disease. Born with a genetic defect related to the development of certain types of white blood cells, the children were vulnerable to severe, chronic infections and would probably have died young. The experiment was an attempt at a new solution - gene therapy. Doctors inserted properly functioning genes into the children's own dysfunctional sequences. At first, it worked wonderfully: nine of the eleven children developed normally functioning immune systems. But within a few years, three had developed leukemia. What had gone wrong?

As it turned out, it was not enough to insert the good genes - it also mattered where they landed. In three of the eleven cases, the new gene's location triggered another gene, an event that led to the leukemia. It was a horrific example of how imprecise targeting could have catastrophic real-world results. In response to the French experiment, the US Food and Drug Administration placed a temporary "clinical hold" on...