In 1990, Ashanti DeSilva was a four-year-old with a rare, life-threatening immune disorder known as ADA deficiency, which made her vulnerable to even the mildest infections (it is also often referred to as severe combined immune deficiency, or SCID—in which the immune system lacks the ability to produce an important enzyme called adenosine deaminase). Doctors then removed her white blood cells and replaced them with genetically altered white blood cells—thus essentially altering the structure of her DNA. Although the treatment proved to strengthen her immune system, the treated cells failed to produce additional healthy cells. In order to maintain normal levels of adenosine deaminase, DeSilva—who is relatively healthy at this writing—still has to get periodic gene therapy to maintain the necessary levels of the enzyme in her blood. But because she also receives doses of the enzyme itself (called PEG-ADA), it is unknown whether or not the gene therapy would have actually worked if it was the only therapy administered.

Such genetic therapies still have a long way to go. And although scientists have made thousands of gene therapy attempts since DeSilva’s time, many treatments have failed to correct the disease being treated—and some have also, unfortunately, caused other diseases.