David A. Roth, assistant professor of medicine at the Harvard Medical School, announced Monday that he and his team of researchers has successfully used gene therapy to improve the prognosis of patients suffering from hemophilia A.

About 13,500 Americans suffer from hemophilia A, the most common form of the disease, which prevents the body from properly clotting blood.

Roth and his team used a non-viral method to introduce factor VIII, the blood clotting protein that type A hemophiliacs lack, into the patients' plasma.

"These are the first positive results from a trial of gene therapy for hemophilia A," Roth said, according to The Boston Globe.

In the experiment, sponsored by Transkaryotic Therapies Inc., Roth and his research team harvested skin cells from each of six test patients, added DNA for Factor VIII to these cells and surgically implanted the cells into each patients' abdomen. At least four of the six patients improved their prognoses.

Gene therapy has been controversial since Jesse Gelsinger, an 18-year-old who suffered from a liver disease, died in a University of Pennsylvania experiment which used viral vectors to introduce genes last year.

Although there are viral methods of gene therapy completely unlike the kind used on Gelsinger, and each method has its own benefits, the non-viral technique used in the study has several advantages, said Gilbert C. White II, the director of University of North Carolina- Chapel Hill's Hemophilia Diagnostic and Treatment Center.

"It has advantages in that you don't have to give the viral vector to the person....You can circumvent some of the problems that might arise due to virus," said White.