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Medical researchers in Britain have successfully treated six patients suffering from the blood-clotting disease known as hemophilia B by injecting them with the correct form of a defective gene, a landmark achievement in the troubled field of gene therapy. Hemophilia B, which was carried by Queen Victoria and affected most of the royal houses of Europe, is the first well-known disease to appear treatable by gene therapy, a technique with a 20-year record of almost unbroken failure.

“I think this is a terrific advance for the field,” said Dr. Ronald G. Crystal, a gene therapist at Weill Cornell Medical College. “After all the hype in the early 1990s, I think the field is really coming back now.”

Gene therapy has had minor successes in very rare diseases but suffered a major setback in 1999 with the death of a patient in a clinical trial at the University of Pennsylvania. Another gene therapy trial treated an immune deficiency but caused cancer in some patients.

The general concept of gene therapy — replacing the defective gene in any genetic disease...