LONDON—A potential cure for children born with a form of an extremely rare immune-system condition often referred to as “bubble-boy disease” has moved a step closer to approval after receiving the support of a European Union regulatory panel.

The treatment, which involves inserting a new gene into the patient’s stem cells, received a positive opinion from the European Medicines Agency’s advisory committee on Friday, paving the way for final approval in coming months.

It is aimed at children with a disease known as ADA-SCID, a rare condition in which a single genetic defect prevents sufferers from developing a robust immune system, leaving them very susceptible to infections. Without treatment, they rarely live for more than two years.

Currently, the best available option is a bone-marrow transplant, but success heavily depends on how well matched the donor is. For the one in four babies who have a well matched family member, a transplant can be an effective cure. But for the rest, success rates can be as low as 50%, according to Bobby Gaspar, professor of pediatrics and immunology at London’s Great Ormond Street Hospital.

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The new therapy, called Strimvelis, was developed by a group of scientists and doctors based in Milan who have used it to treat 22 children over the past 14 years. All are still alive, most without needing any further treatment. In 2010, GlaxoSmithKline PLC struck a licensing deal for the rights to market the therapy.

If approved, it would be only the second gene therapy to be sold in Europe, after UniQure NV’s Glybera for a rare genetic condition in which the body can’t break down fat molecules. No gene therapies are approved for sale in the U.S.

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