The gene editor CRISPR won’t fully fix sick people anytime soon. Here’s why

This week, scientists will gather in Washington, D.C., for an annual meeting devoted to gene therapy—a long-struggling field that has clawed its way back to respectability with a string of promising results in small clinical trials. Now, many believe the powerful new gene-editing technology known as CRISPR will add to gene therapy’s newfound momentum. But is CRISPR really ready for prime time? Science explores the promise—and peril—of the new technology.

How does CRISPR work?

Traditional gene therapy works via a relatively brute-force method of gene transfer. A harmless virus, or some other form of so-called vector, ferries a good copy of a gene into cells that can compensate for a defective gene that is causing disease. But CRISPR can fix the flawed gene directly, by snipping out bad DNA and replacing it with the correct sequence. In principle, that should work much better than adding a new gene because it eliminates the risk that a foreign gene will land in the wrong place in a cell's genome and turn on a cancer gene. And a CRISPR-repaired gene...