WHO spokesman Dick Thompson said an international panel of scientific advisers made the recommendation to the WHO last week.

"The idea was to insert a fluorescent gene in the virus to help speed up screening," Thompson said in a telephone interview.

"The panel ... thought that it was a good idea for several reasons. First it reduces the amount of time that lab workers have to be exposed to the virus," he said.

"Most important, it moves us closer to the day that we can destroy the remaining stock of smallpox."

The gene, called green fluorescent protein, is routinely spliced into organisms ranging from bacteria to mice to make it easier to study various aspects of biology. It is originally taken from a jellyfish.

Smallpox, caused by the variola virus, once killed millions around the world. The virus killed 30 percent of victims and left others scarred for life.

It was eradicated by a global vaccination program led by WHO in 1979 and experts for years debated whether to destroy remaining samples of the virus kept in Russia and the US Centers for Disease Control and Prevention.

But bioterrorism experts argued that several nations had tried to make smallpox into a weapon and that stocks may still exist outside the careful CDC and WHO controls.

Therefore, work proceeds at a very few labs on a new and better smallpox vaccine, and the US government has been working to stockpile supplies of the existing vaccine, which uses a related but far less dangerous virus.

For instance, the US Army Medical Research Institute of Infectious Diseases or USAMRIID works with smallpox virus at its top-security facility in Ft. Detrick, Maryland and the CDC also has a specially equipped lab.

Researchers working with smallpox must work in Biosafety 4 labs, which have special controls to help ensure no one accidentally becomes infected.

But experts agree that the less time spent fiddling with such a dangerous virus, the better.

Thompson said WHO had made no decision on the recommendation and said it would undergo several levels of review.