PROGRESS: Hemophilia may be curable.

After trying for decades to develop a gene therapy to treat this disease, researchers are starting to succeed. In recent experiments, brief intravenous infusions of powerful new treatments have rid patients — for now, at least — of a condition that has shadowed them all their lives.

There have been setbacks — years of failed clinical trials and dashed hopes. Just last week, a biotech company reported that gene therapy mostly stopped working in two of 12 patients in one trial.

But the general trajectory has been forward, and new treatments are expected by many experts to be approved in a few years.

No one is saying yet that hemophilia will be cured. Currently the gene therapy — which uses a virus to deliver a new gene to cells — can only be used once. If it stops working, the patients lose the benefits.

For now, “we are anticipating that this is a once-in-a-lifetime treatment,” said Dr. Steven Pipe, director of the hemophilia and coagulation disorders program at the University of Michigan and a lead investigator of a clinical trial conducted by the biotech company BioMarin.

The successful treatments are so recent it is hard to say how long they will last. But for the few patients who have been through the clinical trials successfully, life after treatment is so different that it’s something of a shock.

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