Why gene therapy is no longer a pipe dream

Last year will be remembered for many things, many of them not good. But here's at least one flicker of hope: 2017 may go down as the year gene therapy finally turned a corner. In late December, the Food and Drug Administration announced its approvalof the third gene therapy since August. These three therapies—two for treating cancer and one for treating a form of congenital blindness—are the first of their kind to pass muster with the FDA. And with hundreds of clinical trials for other gene therapies underway, they won't be the last. For decades, gene therapy was a pipe dream, symbolizing the unfulfilled promises of biomedical scientists who claimed that they could not just treat but actually cure disease by fixing genes. Those promises are finally beginning to be realized.

The logic behind gene therapy is simple. Many diseases are caused by a missing or defective gene; replace the broken gene with a working copy, and you cure the disease. But putting this logic into practice hasn't been so easy. For more than a century...