SAN FRANCISCO - Add hepatitis B to the list of diseases alongside AIDS, herpes and the common cold researchers are using in attempts to create medicines in the sometimes rocky gene therapy research field.

On Sunday, a scientific team of 12 Japanese and Belgium researchers reported it had some success in using a small molecule produced by the hepatitis B virus to temporarily treat hemophiliac mice.

The researchers said their technique more narrowly focuses where the inserted genes land after being injected in the body than the other viruses. Their work was published online Sunday by the journal Nature Biotechnology.

Gene therapy researchers seek to fight genetic disorders by replacing bad genes with functional ones. One of the biggest hurdles in the field is how to deliver the genetic payload into the desired cells without creating other health problems for the patient.

Many researchers are working to replace the lethal genes of viruses such as HIV with beneficial ones and injecting them into the ill. But such engineered viruses sometimes drop their good genes into bad spots in cells, which is what happened recently to two boys who got leukemia after being successfully treated for the immune disorder commonly called "bubble boy" disease.

The researchers in their report said their technique is more focused than others because hepatitis B attacks only liver cells, while other viruses have the potential of entering a wide variety of cells.

The researchers aren't using the hepatitis B virus itself, but instead are inserting disease-fighting genes into "nanoparticles" produced by the virus. Still, those "nanoparticles" carrying blood-clotting genes inserted by the researchers dumped their genetic payload almost exclusively in the liver.

The mice produced measurable amounts of blood clotting proteins for about 40 days, the researchers reported.

The nanoparticle is "a safe vehicle for delivering both genes and drugs," the researchers concluded. The lead author of the study was Tadanori Yamada of Osaka University.

Gene therapy has been a publicly charged topic since the 1999 death of 18-year-old Jesse Gelsinger, who was given a different type of gene therapy for another disease.

French researchers last year rejuvenated the field when they reported gene therapy cured four boys of severe combined immunodeficiency, or SCIDs, and better known as "bubble boy disease." But the excitement was tempered months later because the inserted genes that cured the SCIDS also caused two of the boys to get sick with leukemia.

The researchers said Sunday their technique may better protect against unintended consequences.

"Delivery is the big issue in gene therapy," said Dr. Mark Kay, director of Stanford University's Human Gene Therapy Program."This is encouraging. It's the kind of stuff we need in the field."

Still, Kay said the latest work had limitations, highlighted by the fact the gene therapy stopped working in the mice after a brief period. Continued injections of the new genes will cause the body to build up resistance to the therapy, Kay said.

Another problem arises with the use of the hepatitis B virus because many people are vaccinated against the disease, said gene therapy scientist Inder Verma of the Salk Institute of Biological Studies. The vaccine will kill the therapy as well, Verma said.

"It's exciting and interesting," Verma said. "But now what?"