In 1990, a four-year-old girl with severe immune deficiency received an infusion of her own cells, corrected with a working copy of the ADA gene. It was the first gene therapy trial. The treatment worked, but the field stumbled for two decades. Setbacks, including deaths and leukemia caused by the vectors themselves, slowed progress. Then came successes in inherited blindness, spinal muscular atrophy, and sickle cell disease. Gene therapy is now a clinical reality.

The approach has several forms. Gene replacement adds a working copy of a faulty gene. Gene silencing turns off a gene that is overactive. Gene editing corrects a mutation in place. Cell-based gene therapy modifies cells outside the body and returns them. Each strategy depends on a delivery vehicle, usually a modified virus, that carries the genetic payload into the target cells.

The challenges are substantial. Delivering the gene to the right cells is hard. The immune system may attack the vector or the corrected cells. The effect may fade over time. And the cost is staggering. Some approved therapies list for over $3 million per patient. Health systems are struggling with how to pay for them.

Gene therapy is no longer science fiction. It is a small but growing part of medicine, and it is rewriting what we consider treatable.