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Scientists are edging closer to defeating a longtime enemy of human health: hemophilia, the inability to form blood clots.

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.

No one is saying yet that hemophilia will be cured. Gene therapy — which uses a virus to deliver a new gene to cells — can only be used once. The successful treatments are so recent that it is difficult to say how long they will last. For now, “we are anticipating that this is a once-in-a-lifetime treatment,” said Dr. Steven Pipe of the University of Michigan and a lead investigator of a clinical trial conducted by the biotech company BioMarin.

There are 20,000 U.S. hemophilia patients who lack one of two proteins needed for blood to clot. Virtually all people with the genetic condition are men. Those most severely affected must inject themselves every couple of days with the missing proteins, clotting factor VIII or factor IX. The shots alone can cost as much as $1 million per year. And even with regular injections, people with hemophilia risk uncontrolled bleeding.

A handful of biotech companies are rushing to get their gene therapies to market. Spark, with gene therapy for hemophilia B, and BioMarin, with a treatment for hemophilia A, are starting final-phase clinical trials. Results from the preliminary trials were not perfect. Patients in Biomarin’s trial got, on average, normal or above normal levels of factor VIII in their blood, but in the second year, those levels dropped to a median of 46 percent. Patients in Spark’s trial only reached on average 35 percent of normal blood levels of factor IX. But those levels have remained steady for the two years they have been followed.

But those levels are sufficient for blood to clot, because normal levels are more than people need.