By infusing hemophilia sufferers with safe, redesigned viruses, it’s possible to get their livers making clotting agents that are otherwise missing from their body.

Patients with hemophilia B lack a gene that helps create factor IX, a protein that clots blood, making it difficult for even small bleeds to stop. A promising gene therapy developed by Spark Therapeutics promises to solve that by administering an infusion of viruses carrying the missing factor IX gene. In a handful of patients, early results appeared to be promising (see “Gene Therapy Is Curing Hemophilia”).

Now, a modestly sized trial suggests that Spark may really be onto something. In results published in the New England Journal of Medicine, 10 patients received viruses designed to deliver the missing gene to their livers. Eighteen months after the treatment, on average their livers made 34 percent of the normal level of the clotting agent. That may sound low, but it was enough for nine patients to have no bleeding, and eight didn’t require regular injections of clotting agent.

Some question marks still hang over the therapy. First, the trial is still small, and larger tests will be required before the treatment really proves itself. Another major concern  is how long the effect will last beyond the 18 months of the trial. It’s hoped that it will be a one-time cure, but it might need to be if it’s going to be viable—given the high cost of gene therapies, multiple treatments over a lifetime may not be feasible.